I used to have the formula for crushthrough somewhere, it's fairly complicated. What I can say about it is this, most important is weight of the weapon you are using. Next is damage. The lighter your crushthrough weapon is, the more power strike you'll need to make it effective. WPF isn't in the formula but it does affect damage so it helps indirectly (somewhat). Finally, the weight of the enemies weapon affects their ability to block. So if they're using something heavy and you aren't, don't waste your time.
I don't know if knockdown follows a similar formula or not, my gut says no.