how often can you win (general theory ?)

[pfkaok]

So, mostly this question is for middle or late stages in relation to stack sizes, but i suppose it could apply to early stages as well.

anyways, with N players left (say 5-25% of the starting field), later in the tourney, with blinds large, but not stupidly large such that the average stack is red zone, how often can a good (say 100% ROI) player win when they have an average stack? also, with N players, how big of a stack does a very good player need in order to win 1/N times?

i know of course it will depend a lot on how many players are left, as a very good HU player probably can't win much more than 60% when the stacks are equal and the blinds are fairly large. so thats only 120% of if he won only his share. but say there's 50 players left out of 500. a very good player with an average stack will certainly win more than 1/50 * 120% = 1/42. there must be some equation that takes into account players left that will give a solution to how often you win. does anybody know of such an equation that will at least approximate this?