How do you calculate %xHKO?

That's what I thought was a good way of doing it, but I'm pretty confident that there's an exact way of calculating this in less than O(log N) time.

If it is within my capacity to do, I would like to know how. Also, it is possible to have it work with crits? I know it's hard, as Stereo said, but I'm very much interested in making a hits to kill calc for all most classes.