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Advance prime requisite bonuses?

Some older editions of D&D have a rule where every batch of XP you would get, you get plus or minus a percentage of that XP. For example, a cleric with wisdom 4 gets -20% XP, or a fighter with strength 16 gets 10% extra XP.

Threshold method

I saw a YouTube play where they instead adjusted the XP thresholds by -10% for those high-stat characters so that for a strength 16 fighter, she would only require 1800 XP to advance to level two instead of the normal 2000.

This is wrong. 1800 + 180 is not 2000. Instead, for the threshold method, multiply 2000 by 10/11 and ceil it to find the new threshold, in this case 1819. 1819 + 181 is 2000.

PRB Threshold factor
-20% 5/4
-10% 10/9
+5% 20/21
+10% 10/11

Advance method

Then I saw a houserule PDF that suggested getting the bonuses in advance. That appealed to me more than rewriting the XP curves for every character.

But it has the same math problem which the text acknowledged, saying it’s “within 1%”. Maybe so. But 1800+180 is still short a bandit and a heartbreaker, 10 xp each. That’s the value of two human lives right there. And that’s only at level one. Going from level four to five, you need 8000. 7200+720 is short 80 xp, that’s an entire team of synchronized swimmers!

The advance for level one shouldn’t’ve been 200, it should’ve been 181.

PRB Delta factor
-20% -1/4
-10% -1/9
5% 1/21
10% 1/11

A problem with either method

It can happen (for example through multiple stacked wish spells) that a character’s prime requisite penalty or bonus changes mid level.

The workaround when using the advance method would have to be to subtract the previously awarded advance bonus and then reaward both the full listed percent for the XP already earned since last level, plus the new delta factor for the XP remaining until next level.

For example, I am a level 1 fighter with a 10% bonus having been awarded 181 XP in advance trying to get to level 2 at 2000 xp, and in doing so I earned some more XP so that I’m now at 681 XP. I then get hexed by amazing starships and now my bonus is only 5%. I’d need to first subtract the 181 XP advance gift to undo it, leaving me with 500 XP, and then get my by-the-book old bonus applied to that, so I’m at 550 XP, so my next remaining delta becomes 1450 XP, and my new advance gift becomes 1/21 of that which is 69 XP leaving me with 619 XP on my path to 2000 XP (the threshold never changes with this method).

That’s was cumbersome enough and might be a good argument for not using tricks like these. The workaround with the threshold method is even more difficult, by a lot.

Let’s say I’m a level 1 fighter with a 10% bonus, meaning my threshold has been lowered to 1819 XP. I’ve then earned 500 xp and then the amazing starship is at it again with more hexes more spells so my bonus gets lowered to 5%. That means that I need to calculate out a whole new XP curve where for every remaining level I need to first restore them to the by-the-book thresholds, then subtract 50 from them for the bonus I already earned (10% of 500), then subtract 500, apply the threshold factor, and re-add 500. So the 1819 for level 2 becomes ((2000 – 50 – 500) × 20/21) + 500 = 1881 and I need to do that for every level.

I can see why many editions remove the prime requisite bonus rule, but I’m considering these shortcuts (and leaning towards the advance method if any) with the updated delta factors here.