Crushthrough is also pretty retarded as a game mechanic, the way its implemented
If we're just talking about knockdown though, i'd make the chance 0% until a certain condition is met and then make it 100%. So its decided by the players actions what happens rather than a dice toss by the server. So if the damage is over 40 for eg it is auto knockdown, or if you hit the head, or if its a back hit. Stuff like that that can be countered rather than just random factors. Missing 1 block should not put you out of the fight against some shitty 1 hander