1/10th of what you think your opponent will pay you off with if you hit your set. If the guy is a complete rock and won't pay off any suspicious raise, tone it down a notch. If the guy will put his entire stack in with AA or KK, regardless of the board and your betting patterns and you're sure he has AA or KK, then call up to 1/10 th of his stack.

Reason being you have a 1 in 8 chance of hitting that set, so when you do, you wanna get paid enough to offset you calling with PP preflop. I say to only call 10% because calling 13.5 (or whatever 8/100 is) would only be breakeven; I'd like to get paid a little bit more. )