Actually, don't power attack at all. The optimal power attack strategy for a single attack isXCorvis said:
Damage: 1d8 + 6 (+4 str, +2 Spec) x3 for charging, or 3d8+18.
Don't forget to power attack if you're human.
pa = (21 - tn - dmg)/2, where
tn = natural number needed to hit
dmg = average damage on the attack
pa = points of power attack to use. If x.5, rounding up or down makes no real difference.
Since you are doing minimum 21 damage, using power attack reduces your average damage, unless tn < 2, in which case you should use enough power attack to bring tn to 2.