The TL;DR of this guide: Using basic calculus to find the maximum of damage as a function of the Power Attack penalty, I derive equations for how much damage Power Attack adds. I also show that the case with multiple attacks at the same attack bonus with a two-handed manufactured weapon is the best possible case for Power Attack and that with BAB-based iteratives, TWF, or natural weapons, Power Attack adds less damage. In the best possible case, once you have more than 20 damage from other sources, your hit chance needs to be off the RNG, at the maximum of 95%, to get more damage from Power Attack than you would from Weapon Specialization. With some reasonable assumptions about damage from magic items and Strength, anything that adds damage without attack bonus makes Power Attack useless. Power Attack can add damage when you're below a certain level and have certain abilities or effects that add attack bonus and damage, though only with weapons with small enough damage dice; with abilities, effects, or circumstantial modifiers that increase hit chance but not damage; against enemies with unusually low AC; and in combinations that increase its multiplier, remove the attack penalty, or both.
The Mathematics of Power AttackThe discussion thread is here:
http://www.minmaxboards.com/index.php?topic=12230.
