See Lou Krieger's article:.
http://www.pokerhelper.com/forums/Pot-Odds-Poker.php

His formula and chart are pretty simple to use. I also try to factor in my position and how many hands folded early. You pretty much have to go with the odds based on a 52 card deck and the 5 you know about (at the flop), but I will make slight adjustments. If there were a number of folders before the flop, then I will discount a straight draw that involves cards between 2 and 9, or if I need over cards and it is a family pot, I have to factor in that some of the cards I need are already in play.

You can figure general odds, but there is no way to be dead-on because you don't really know what cards are still in the deck. In the end, you can correctly figure you are a 91% favorite at the turn and be up against a flush draw, a straight draw and some guy with pocket deuces trying to make trips. It's like dancing in a minefield- No matter how good you are you'll still get blown up quite often.

Koroshiya.
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