Ok, here's a statistical analysis of the chance to damage based off the % chance to hit and % chance to damage, assuming that total defense + Toughness = 20, and Attack + Damage = 20. This ignores critical hits, which would raise the numbers slightly.
Quick breakdown of how I got the number.
Min 5%, Max 95% of any individual %, as a 1 always fails, and a 20 always succeeds.
Chance to hit/Attack = 5% * ( (20 + Attack Bonus) - (Damage Bonus + 10) + 1)
20 = highest roll, 10 = base defense.
Basicly, for every number 20 or below you can roll and still hit, you have a 5% chance to hit. So, the difference between that total defense and the max attack +1 (to include when they equal) gives you the total base chance to hit.
If hit:
Chance to Resist/Hit = 5% * ((20 + Toughness) - (15 + Damage) + 1)
Chance to Damage/Hit = 100% - Chance to Resist
20 = highest roll, 15 = base damage save DC
Basicly, for every number 20 or below you can roll and still hit, you have a 5% chance to damage. So, the difference between that total damage and the max toughness +1 (to include when they equal) gives you the total base chance to resist damage. The chance to damage is the other option, so you subtract the resist chance from 100%.
So,
Chance to Damage/Attack = Chance to Hit * Chance to Damage/Hit
Basicly, we're taking the % chance to damage per hit of the chance to hit per attack.
Here are the results. (Note, this does not add any other modifiers.
Code:
defense/Toughness
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 20 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
1 19 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
2 18 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
3 17 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
4 16 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
A 5 15 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
t 6 14 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75% 4.75%
t 7 13 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75% 4.75%
a 8 12 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75% 4.75%
c 9 11 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75% 4.75%
k 10 10 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50% 4.75%
\ 11 9 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25% 9.50%
D 12 8 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00% 14.25%
a 13 7 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75% 19.00%
m 14 6 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50% 23.75%
a 15 5 4.75% 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50% 28.50%
g 16 4 4.75% 4.75% 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00% 31.50%
e 17 3 4.75% 4.75% 4.75% 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00% 34.00%
18 2 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50% 36.00%
19 1 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50% 37.50%
20 0 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 9.50% 14.25% 19.00% 23.75% 28.50% 31.50% 34.00% 36.00% 37.50% 38.50% 39.00% 39.00% 38.50%
So, what do those numbers mean? Well, the best you'll ever do as far as chance to damage per attack assuming all are maxed out at your current power level on both sides, is when your attack bonus is 1-2 points above your opponents defense bonus, while your damage bonus is 1-2 points below your opponents toughness save.
I don't see a huge variance with a +5/-5 tradeoff, and it can be potentialy detrimental. The wose case, is someone who took one +5 Attack/ -5 Damage attacking a -5 defense/ +5 Toughness has only a 19% chance to do damage, or someone who took -5 Attack/ +5 Damage attacking a +5 defense/ -5 Toughness has only a 4.75% chance to do damage, but those are the extremes.
Those numbers should play out at higher or lower numbers, so I think a flat +5/-5 is ok, or +4/-4 for for PL 8 and 9 and +5/-5 for PL 10+ is also fine, and keeps some sanity on the lower ends.