I agree with Jeff. The best is trying to determine a nice amount that will make flush drawers writhe in agony while pondering that call. Ideally, you would like just beyond value, so they have to accept the fact that to make this call that is just a bit beyond value, they're making a decision that favors you. Anyone with TPTK or 2-pair will call instantly, and you want that action since they have so few outs for hitting their set or full house.

This is a good situation where lesser players will likely give you tells about their hand. If they spend time pondering this call and re-checking their cards and the board, then you know for certain you have them on a draw and if you see that third spade hit on the turn, it is time to lay that set down. Same thing goes for Mr. TPTK/2-pair. If he makes his call very quickly, you can probably assume that he thinks his hand is good. Look out for any re-raises from him in the future.