It's not that easy and depends on the other stacks. Also, all models are imperfect etc. I think the best way is to calculate the odds that another player comes in first, then calculate the odds that you come in first in the tournament if they weren't there, repeat for each player, and take the sum. That sucks though.

You're probably better off just using the ICM calculator, which gives you EV instead of %s by places--that's probably better for you. It's linked to in the fact.