0
However...
That answer is assuming that you were asking if there were any prerequisites, if you mean equivalent in damage:
damage=base_damage*(0.85+wpf*0.0015)
So every 100 points of wpf will add 15% to your damage, but the base damage is 85% with no extra wpf, so besides the first 100 to get it to "normal" you need an additional 53 points to get it to 108% damage (like you had an extra Powerstrike), so:
For damage equivalency:
Base ≈ 100 wpf
1PS ≈ 153 wpf
2PS ≈ 207 wpf
3PS ≈ 260 wpf