Page 1 of 1

Minimizing math while also balancing the "Siege Engine"

Posted: Wed Nov 27, 2019 11:48 am
by Noldor
Hello all!

Edit : This post covers levels 1-12. A later post in this same thread covers Levels 1-22. ;)

Introduction :
I am an experienced old-school DM/GM and i have been running 1st edition campaigns for about 26 years. I have recently started playing C&C and i am very happy with the game. It gives me the feel of 1st edition that i need while at the same time giving my players a system that feels more modern. Its the perfect compromise. Its a bonus that it works with all of my old material.

I only had 1 issue with C&C and i feel that i have a great option and now i want to share that with whoever is interested. :)

Background :
I do not like picking an arbitrary number for a challenge. I did not find a good balance with the 18/12 split with a CL of 0-10. Either it was to easy or it was to hard and it felt arbitrary. The CL i chose would affect the outcome by 0-50%. That makes me feel that i decide the success of a task more than the rules/dice. I also have a hard time picturing challenges as levels or adversaries. I love the core concept but i would like more defined challenge levels.

Way forward :
I am much more comfortable with picking a difficulty level by name than by number. Having those names converted to static numbers also meant that it would be possible to write an adventure with challenges defined by names and having them work with characters of different levels without modification.
So i decided upon Normal, Hard and Heroic difficulty. I skipped "Easy" because no roll should be needed for an "Easy" task. ;) The variation in the chance of success comes from a combination of one of these "difficulty levels" and the "attribute bonus" and "level" of the character. I will supply a pre-calculated "Challenge Class" for each of these three "difficulty levels" depending on the party level in a table bellow.

Normal : This would be the common challenges. Things like picking a lock, hiding in shadows in a dungeon, pushing a heavy boulder or deciphering a language with the same origin as ones own.

Hard : This would be the more uncommon challenges. Like picking a high quality lock in dim light. Hiding in shadows in a well lit hallway. Forcing open a barred door. Deciphering a forgotten language of unknown origin.

Heroic : This would be the rare heroic moments. The long-shot heroic deeds that can change the fate of the campaign. This would be picking a lock made by a master locksmith. Swinging on a rope across a chasm at the same time as grabbing a thrown item in mid air. holding back the trapped ceiling that is otherwise slowly going to crush the party.

I did the math and could not make these three levels of difficulty line up so that the level of the character and the prime vs non-prime attributes balanced out in a good way. So i switched to a "Challenge base" of 16 and a +4 bonus for Prime attributes to even out the statistics and fill the "gap". This worked wonders for the math and for the difference between prime vs non-prime attributes. I kept the assumption from the CKG that at level 1 with a 0 attribute modifier, a prime attribute should have a 50% chance at a "normal" task and the non-prime attribute should have a 20% chance at that same task (it came out to 30% for me).
I also made sure that the characters level improved the chances of success at a steady pace without outpacing the challenges and without the CL going in lockstep with the characters level eliminating the feeling of character improvement.

The best thing of all! At the start of a session i just have to make ONE single lookup. I lookup the average level of the party on my small table and i get three "Challenge Classes" (challenge base + challenge level) already calculated for each of these three difficulties. So when we play i just have to decide if the task is Normal, Hard or Heroic and that gives me the exact number needed. The player rolls 1d20 and adds +4 for prime, attribute bonus and level and i just compare the result to either the normal, hard or heroic number. :D

So to get to the point. Here is the finished table that i use in our campaign. At the start of the session i only grab the three numbers for the average level of the party and off we go. No calculating "Challenge Class" at the table, no 12 or 18, no nothing. Just roll vs the number of the corresponding difficulty. Whenever needed i can still use the level of an NPC as a CL using a CB of 16 if i want to. All other saves and checks from the Rules are unchanged (using a CB of 16 and a prime bonus of +4).

To use this table you would have to use the "challenge base" of 16 with the +4 bonus for a prime attribute and +0 for a non-prime attribute.
The "challenge base" is not needed. The "challenge classes" come pre-calculated in the table bellow.

Image

For me. Running C&C became easier then ever and it feels less arbitrary and more fair. A hard challenge is always hard while at the same time letting characters progress in skill. I don't have to do any math for each attribute check. I don't have to know if the attribute was prime and i don't have to know the value of the CB or the CL.
All i need to know is : Average party level is 4, Normal = 17, Hard = 20, Heroic = 23.
That is all i need during play for everything to balance out the way i want.

An example of what i would have in front of me during a typical session of our C&C campaign. ;)
Image

To back up the validity of my chosen solution i provide my math below. It shows how the characters chance of success improve as they gain levels. It shows the effect of attribute modifiers on these chances and it shows the difference between prime and non-prime attributes.
This massive table is ONLY supplied for showing the underlying math. This is NOT needed during play. Only the above list of level vs difficulty is used during play. ;)
Image

The math behind this solution assumes that characters are retired after level 12. We don't play characters of a higher level. I have posted a similar table in this thread that covers levels 1 - 22 for anyone running a campaign with a wider span of levels.

Thanks to TLG for a great game and thanks to the great community.
I hope this helps anyone that feels the same way that i do.
Happy gaming. ;)

Re: Pre-Calculated "Challenge Class". Easier task resolution

Posted: Wed Nov 27, 2019 12:24 pm
by finarvyn
Nice analysis of the numbers, and welcome to the boards! :D

Re: Pre-Calculated "Challenge Class". Faster task resolution

Posted: Wed Nov 27, 2019 3:24 pm
by Noldor
Thank you Finarvyn. :D

It seems like a wonderful community.
I thought that i would take this chance to also show how our custom character sheet works in tandem with the above system. Our sheet lists the characters attributes and attribute modifiers as is common on most sheets but it also has a "check" value for each attribute, right there on the sheet. These are pre-calculated modifiers for attribute checks. I think i have seen this on other sheets available online and it works wonders together with the simple system above.

The player only needs to add 1 number to his roll and the CK just uses the number of the chosen difficulty without any calculation required. We feel that it works out great. Everything runs so smooth. :D

So when a player wants his character to do something like, tipping over an old column to slow down the approaching Orc hoard. I as the CK could decide that that is a Hard challenge (19 for level 1). The player would then roll 1d20 and simply add the bonus in the "check" field for his characters STR attribute without having to make any further calculation.

One roll, adding One number and comparing to One pre-calculated number. Could not be simpler. And all without modifying the "Siege Engine" in any major way and keeping compatibility intact.

Image

Re: Pre-Calculated "Challenge Class". Faster task resolution

Posted: Thu Nov 28, 2019 10:34 am
by Noldor
I made another version of the table that we use. This one spreads the "challenge levels" across 22 levels instead of 12.
This was made to make this method of three pre-defined difficulty levels available to CKs running campaigns with a wider span of levels.

This version also requires the use of a "Challenge Base" of 16 with a +4 Bonus for Prime attributes. ;)
Here is the table of pre-calculated "Challenge Class" values for level 1-22. Used during play. :D
Image

Here is the huge table with the underlying math. This is NOT needed unless you are curious about the math behind the numbers in
the table above. ;)
Image

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Fri Nov 29, 2019 11:10 am
by soloplayer
Cool, I like your style...

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Fri Nov 29, 2019 12:50 pm
by Noldor
Thank you soloplayer. I am glad you like it.
I hope as many people as possible will find this useful. I would be glad to generate an additional level 1-22 table for anyone who feels that my table has to little challenge or is to "easy".

While running my campaign i never ever fudge a result and i never hold back on consequences of a failure. But i also always give the characters some kind of chance for success. So i find these tables perfectly balanced but if anyone would like to use a table with more challenging values without having to do all of the maths i could make one.
When i made the level 1-22 table i finalized my excel sheet so that it automatically generates all of the % statistics.

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Mon Dec 02, 2019 10:12 pm
by Captain_K
Very cool bits of math and helpful. I'm in your camp fully, I like numbers.... and things get wonky as you go up in level I fear... consider pulling this together and publishing it in the Domesday magazine? send me a PM and I'll shoot you some emails to the editor to submit and get your great work published to a broader audience and longer lasting storage

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Sat Dec 07, 2019 3:58 pm
by jeffb
This is excellent. Thank you for the time and effort Noldor! Totally stealing this. Should be in the next print of the CKG ;)

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Sat Dec 28, 2019 10:10 am
by dawnrazor
Thanks and an interesting concept. However, I'm a bit confused here - are you suggesting a "roll under"-mechanic here? If not, things get harder the further you progress in level... Or am I misinterpreting something?

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Sun Dec 29, 2019 2:05 pm
by Noldor
dawnrazor wrote:Thanks and an interesting concept. However, I'm a bit confused here - are you suggesting a "roll under"-mechanic here? If not, things get harder the further you progress in level... Or am I misinterpreting something?
I am sorry, the prerequisite for understanding my original post is an intimate knowledge of the C&C rules. I am not suggesting any new mechanic or any real change to the C&C rules. I am only supplying a table with pre-calculated and pre-balanced challenge classes (CC). You still roll high as in the default C&C rules. The only alteration would be a constant challenge base of 16 and a +4 bonus when using a "prime attribute".
Noldor wrote:I also made sure that the characters level improved the chances of success at a steady pace without outpacing the challenges and without the CL going in lockstep with the characters level eliminating the feeling of character improvement.
This is all illustrated in the huge table of numbers i posted on 27 nov. It shows the percentage chance of a certain task dependent on character level, prime attribute and attribute bonus. ;) It is not needed to play but only to illustrate how characters of a higher level have a higher chance of success while still facing a fitting challenge for a Normal, Hard or Heroic Task. :D

The "challenge class" of tasks would have to increase as the characters level increases (This is implicite at the core of the default C&C rules). The challenge should not increase in such a way that it nullifies the characters level progression but also not in a way were any task that requires a roll is a guaranteed 100% success. Rising "challenge level (CL)" for tasks is at the core of the default C&C rules. What i have done is just, prepcalculate what would be fitting to use as a normal, hard or heroic task for a character of a certain level. Eliminating the need of making these calculations at the table. If you inspect my table more closely you will find that the characters level go up faster than my pre-calculated challenge classes. ;)

For example (Using my table):
Two characters. One character of level 1 and one character of level 7 each face a Hard task. They are both using a "prime attribute" in their check.
(They are facing different tasks that would be considered "Hard" for that specific character at that specific level, as decided by the CK)
The level 1 character would have to beat 19.
The level 7 character would have to beat 23.

The level 1 character would roll 1d20 and add 4 (prime) and 1 (level). possible results : 6-25
The level 7 character would roll 1d20 and add 4 (prime) and 7 (level). possible results : 12-31

Therefore :
The level 1 character would have a 35% chance of beating 19.
The level 7 character would have a 60% chance of beating 23.

Above you can see that a Hard task becomes more easy as the characters level increase. So even if the "Challenge Class" is a larger number for a "hard task" at higher levels the character still has a greater chance of success. :D

Keep in mind that in this example the characters use a prime attribute. If the attribute was not prime their chances of success would only have been 15% for the level 1 character and 25% for the level 7 character. ;) The whole point of the table is that you don't have to calculate all of this. I am just supplying this example to clarify why the "challenge class" increases as the characters level increase. You just need the three values for the avarage level of your player characters and then you can rely on the Normal, Hard and Heroic "challenge classes" being balanced for that whole session. (You can use the individual level of each party member but i use an avarage and that works fine. The right character for the task. ;) )

If the avarage level of the party is 6 you only need to know that the numbers to beat are :
Normal : 19
Hard : 22
Heroic : 25

Dont get confused by all the maths and the large table full of numbers. All you need is this. ;)
Image

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Thu Jan 02, 2020 3:11 am
by Captain_K
General questions why does challenge level go up with level? low level do not attempt heroic and heroic high level PCs do it all the time... that's why they're heroes...

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Thu Jan 02, 2020 4:11 pm
by serleran
This statement and therefore the entirety of the premise is walking a slippery rope; it is partially true but not entirely true: "The "challenge class" of tasks would have to increase as the characters level increases (This is implicite at the core of the default C&C rules)."

The "challenge class" increases only when the opposition is harder than it was before. A 3 HD orc casting a burning hands spell is going to be a 3 (barring other modifiers the CK throws in for whimsy) regardless of what level the characters are when they encounter it. Likewise, simply because you come across a lock or trap at level 20 does not mean the challenge for that lock/trap is 20. The difficulty does not increase at an arbitrated mandatory rate but it is suggested that it remain +/- 6 of current level (even then, this is not always even possible -- 8th level party vs. 1 HD skeletons, for example.)

Otherwise, I suppose good job if it makes things easier.

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Fri Jan 03, 2020 1:40 pm
by Noldor
Captain_K wrote:General questions why does challenge level go up with level? low level do not attempt heroic and heroic high level PCs do it all the time... that's why they're heroes...
The name of the "difficulty levels" have no correlation with the status of the PC. Its just a name so that the CK can ask himself if the task before the PC is normal, hard or heroic. Heroic is just a name for the level of difficulty. The names could just as well have been "normal, hard, longshot" or " normal, difficult, impossible". ;) The PC facing the challenge might be low level or high level and the CK can then judge if the task is normal, hard or heroic. If you look at the statistics i posted earlier you will see that the PCs chances of success go up as they gain levels. The table is balanced in such a way that normal, hard and heroic tasks stay normal, hard and heroic while at the same time increasing a PCs chance for success as that PC levels up.
The CK would just ask himself "is this task normal, hard or heroic for this character in this situation". The table takes care of the calculations.
serleran wrote: The "challenge class" increases only when the opposition is harder than it was before. A 3 HD orc casting a burning hands spell is going to be a 3 (barring other modifiers the CK throws in for whimsy) regardless of what level the characters are when they encounter it. Likewise, simply because you come across a lock or trap at level 20 does not mean the challenge for that lock/trap is 20. The difficulty does not increase at an arbitrated mandatory rate but it is suggested that it remain +/- 6 of current level (even then, this is not always even possible -- 8th level party vs. 1 HD skeletons, for example.)
I get a feeling that might not have read the original post in this thread? This table is not intended for use in combat or other situations predefined in the C&C rules. A 3HD orc casting burning hands will still be level 3. This table does not replace any rules in C&C. Saves and combat and other rules stay the same. This table is only calculated for "on the spot" task resolution (Navigation, jumping pits, picking locks, forcing doors, decoding ancient runes and so on) Tasks do not get more difficult as the chracters level up. If you look at the supplied statistics the tasks actually get easier as a the characters level up (i posted examples on Nov 27). When we play RPGs every challenge in the world does not have a pre-set challenge rating as in an MMO. A challenge can not get arbitrarily harder because it was not defined before it turned up. Situations appear dynamicaly and the CK has to decide how difficult a task is to accomplish. These unexpected challenges should increase in difficulty but not overshadow the level progression of the character. This is what i have precalculated. If the CK consideres the task easy for a character of a certain level then no roll would be needed. (no fun rolling for a task with an automatic success or what has otherwise become a mundane task for the PC)

Without my table the characters face an unexpected task and you would have to imagine that task as an opponent with an arbitrarily choosen level of about 0-10 (cl) and then you would add that value to either 12 or 18 (cb) depending on the attribute used to get a challenge class (CC).

With my table the characters face an unexpected task and you would just have to ask yourself if the task is of normal, hard or heroic difficulty for that character in that situation. ;)

* Looking for hidden traps might be hard for a Cleric while it could be a normal task for a Rogue.
* Jumping a chasm and grabbing a rope to swing across might be a hard task while jumping out into darkness and grabbing an oiled rope might be an heroic task. ;)
* Navigating through an unknown forest could be a normal task but if a thick fog rolls in it might turn into a heroic task.
* Swiming across a lake would probably not need a roll while swiming in chain armor trying to escape an alligator might be a hard task.

My precalculated table takes into account the level of the characters and gives you a fair challenge. This would all be up to CK judgement and i find it a lot easier to ask myself if the task is normal, hard or heroic then to ask myself if the difficulty is a number between 1-10. Using the table i always present a fair challenge that keeps the game exciting and fair. An unexpected challenge is never to easy or to hard by accident and i don't have to make any calculations or make up any arbitrary numbers.
Picking an arbitrary number between 0-10 changes the challenge difficulty by 0-50% which has a bigger effect on the chances of success then attribute bonuses (and level at low level) with a number the CK just pulled out of thin air.

These tables are just supplied as a tool for quick, easy and ballanced play without any need for randomly choosen numbers or doing mathematical calculations during play. Just pick the difficulty of any unexpected task, roll and move on with the story. :D

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Fri Jan 03, 2020 6:15 pm
by serleran
I think you're confusing something, or perhaps I am more stupid today -- both seem likely.

Your chart (the one stated as "the only one needed") indicates that level 13 has a 24 normal difficulty. If that "normal" were a challenge of 1, the task is automatic (if Prime), noting that it may be automatic earlier based on additional modifiers (this is assuming so far there is no attribute adjustment, for example) -- a 1 falls within the defined range of "normal." I also see a reference that a roll of 1 is a failure and a roll of 20 a success -- this is not true in C&C.

But, doesn't really matter.

If your chart works, great.

I don't find it hard to add.

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Fri Jan 03, 2020 7:26 pm
by Noldor
serleran wrote:I think you're confusing something, or perhaps I am more stupid today -- both seem likely.

Your chart (the one stated as "the only one needed") indicates that level 13 has a 24 normal difficulty. If that "normal" were a challenge of 1, the task is automatic (if Prime), noting that it may be automatic earlier based on additional modifiers (this is assuming so far there is no attribute adjustment, for example) -- a 1 falls within the defined range of "normal." I also see a reference that a roll of 1 is a failure and a roll of 20 a success -- this is not true in C&C.
See, this still makes me think that you didn't read the original post carefully and skipped the statistics. :) The only two stated modifications of the C&C rules are a single "Base challenge" of 16 with a +4 bonus when using a prime attribute and automatic fail on 1 and success on 20.
I am not sure how you interpreted this when you say : " If that "normal" were a challenge of 1" ??
The table says that if your PC is level 13 then 24 would be the number to beat when the CK feels that your PC is facing a normal challenge. There is no additional numbers or calculations so i don't know what you mean with a challenge of 1?
A normal challenge of 24 at level 13 gives the PC a 70% chance of success (using prime) as shown in the huge diagram of statistics posted above.

Ex : Roll 1d20, add 13 (your level) and +4 (using prime). That gives a results of 18-37. That means that you would fail that roll on a result of 1-6 on that d20. (30% chance for fail and 70% chance for success. Results of 18, 19, 20, 21, 22, 23 would be fails) ;)

This way there is no need to give an unexpected challenge an arbitrary challenge level taken from thin air. There is no need to use a different base depending on prime or non-prime attribute. Just use the pre-calculated number and you are guaranteed a fair challenge that would be of either normal, hard or heroic difficulty for that character. :D
If you prefer making up a number, adding that to another number that is great. This is just an easier/faster option. I have not made any major changes to any rules. These are just numbers that have the Base challenge (BC) and Challenge level (CL) already calculated into a final Challenge class (CC) that would be appropriate at specific levels. I just made three for each level so that the CK easily can judge if the challenge is normal, hard or heroic.

I hope this clears things up?

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Sat Jan 04, 2020 9:03 pm
by Captain_K
OK, I get that now... I just had in my mind that normal was normal for 1st level... and thus would be almost automatic for high level... heroic for first level would be almost impossible AND much easier with level increases... I had the "challenge" and their names being fixed and only level increasing...

Re: Pre-Calculated "Challenge Class". Faster task resolution

Posted: Fri Jan 10, 2020 11:42 am
by JShan101
Very intrigued by this. One thing to clarify...on the chart below, is level (I'm assuming 1st?) already added in? So the +6 for a STR check is +4 for prime, +1 for the attribute bonus, and +1 for level, correct?

Nice work, by the by. This may find a home in my game very soon!
Noldor wrote:
Image

Re: Pre-Calculated "Challenge Class". Faster task resolution

Posted: Mon Jan 13, 2020 9:53 am
by Noldor
JShan101 wrote:Very intrigued by this. One thing to clarify...on the chart below, is level (I'm assuming 1st?) already added in? So the +6 for a STR check is +4 for prime, +1 for the attribute bonus, and +1 for level, correct?

Nice work, by the by. This may find a home in my game very soon!
Noldor wrote:
Image
That is correct. +4 for prime, +1 for attribute bonus and +1 for level. This picture is a small part of our character sheet holding the characters attribute values. We revise the character sheet every time the character gains a level. This way we have these numbers already prepared when a roll is needed and the game flows much smoother. Simply roll 1d20, add the "check" value for the attribute and compare to the pre-calculated difficulty picked by the CK for the task at hand. ;)
In my experience it is very rare for a character to attempt a task that excludes the characters level from the roll. As a CK i usually just use a harder difficulty from my pre-calculated table instead. So for a task where a rogue needs to beat a normal difficulty, a cleric might need to beat a heroic difficulty.
Example : A ranger using wisdom, attempting to track a band of Orcs through a forest might use a normal difficulty while a cleric (wis prime) attempting the same task might use a hard or heroic difficulty depending on CK judgement. (The CK might even decide that the task is impossible for the cleric ;) )

This avoids a lot of the questions and math during play. :)

Instead of : "Make a strenght check" Rolls 1d20, announcing "Strength check prime", adding +1 for an attribute bonus and asking if the task entitles me to add the level of the character and then comparing to either 12 or 18 plus any challenge level chosen by the CK.

We use : "Make a strength check". Rolls 1d20 adding +6 (check) and comparing to a pre-calculated value chosen by the CK. (normal, hard or heroic)

And remember. This method does not replace any rules. These are just pre-calculated challenge classes (CC) that present a fair and balanced challenge for a character of any level. They are calculated using the rules in the CKG and PHB. I only use these values for task resolution or improvised actions and events. Saves, combat, magic and other rules in C&C remain unchanged (except for using a single base of 16 with a prime bonus of +4)
I am very happy that you like my method of play. It is presented here to help anyone who feels the same way that i do and want to simplify the flow of the game. I hope it works as well in your game as it has in mine. :)

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Mon Feb 10, 2020 4:08 pm
by mordrene
I'm not going to lie, this has a hip beat and I can dance to it!

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Thu Feb 27, 2020 9:04 pm
by Cal_413
Hi, I think your method is very interesting. I just started to learn the rules of C&C and I think that the different challenge base for prime and secondary is more confusing than a single base challenge plus a bonus for prime and no bonus for secondary.
My question about your simplification table is why you choose a base of 16 and a +4 bonus instead a base of 18 with a +6 bonus for prime? If would be more accurate with the SIEGE engine.
Sorry for my English!

Re: Minimizing math while also balancing the "Siege Engine"

Posted: Fri Feb 28, 2020 9:54 am
by Noldor
Cal_413 wrote:Hi, I think your method is very interesting. I just started to learn the rules of C&C and I think that the different challenge base for prime and secondary is more confusing than a single base challenge plus a bonus for prime and no bonus for secondary.
My question about your simplification table is why you choose a base of 16 and a +4 bonus instead a base of 18 with a +6 bonus for prime? If would be more accurate with the SIEGE engine.
Sorry for my English!
Hello Cal_413. Happy to hear that you like the simplicity of my way of using pre-calculated difficulty values while playing CnC. I hope you find it as useful in your game. :)
Now let me quote myself from my original post. ;)
I did the math and could not make these three levels of difficulty line up so that the level of the character and the prime vs non-prime attributes balanced out in a good way. So i switched to a "Challenge base" of 16 and a +4 bonus for Prime attributes to even out the statistics and fill the "gap". This worked wonders for the math and for the difference between prime vs non-prime attributes. I kept the assumption from the CKG that at level 1 with a 0 attribute modifier, a prime attribute should have a 50% chance at a "normal" task and the non-prime attribute should have a 20% chance at that same task (it came out to 30% for me).
Reading several house-rules for CnC in different forums and blogs it seems common for people to use a single "base challenge" with a bonus for prime instead of using two different "base challenge" values depending on "prime" or "non-prime". Another thing that seems common is to lower the gap between prime and non-prime. For me it was a matter of carefully raising the CL (challenge level) for each named difficulty as the characters progress in level to keep the challenge balanced without non-prime lagging behind. This was easier with 16 +4.
I wanted my game balanced to avoid scenarios were it is pointless to even try if your attribute is non-prime, and almost an automatic success if your attribute is prime. Avoiding this was accomplished by bringing them together by two points (10%). Prime still has a huge advantage but non-prime gets a slim chance to save the day if needed.

This is mostly a matter of taste but altering the base to 16 with a bonus of +4 for prime, is 100% compatible with the rest of the "Siege-engine".
I don't see any point in striving for "accuracy" instead of balance if you retain compatibility either way. ;)
One could create a list of pre-calculated difficulty levels just like mine using 18 +6 instead of 16 +4 and it would work just as well. It would just have a different balance between prime and non-prime. :)

As an additional boon (that i don't value that much myself) 16 +4 is lower than 18 +6 which makes the maths slightly easier for some gamers during play. ;)