Something like: CON/(CON+WILL) - 10% chance to INCREASE the damage
WILL/(CON+WILL) + 10% chance to DECREASE the damage
Someone with 10 CON e 10 WILL = 10/20 - 10% = 40% chance to INCREASE the damage | 10/20 + 10% = 60% chance to DECREASE the damage;
Someone with 700 CON e 30 WILL = 700/730 - 10% = 85,8% chance to INCREASE the damage | 30/730 + 10%= 14,1% chance to DECREASE the damage;
Maybe you can still do it, and just make the soap increase even more the base chance to decrease the damage
