The fact is that what your opponent holds does affect your odds, but there is no way to figure what they hold. It's like happy says. Any math you come up with will figure on whether they have it in their hand and if it is not in thier hand. The poker gods don't eveen try to take this into account, they just figure odds on the cards not in their hand. Basically if the table is 9 handed that's 2 cards in your hand, 3 on the flop, and 47 available. Deal 20,000 hands and the odds will come up the same. For example, a four flush draw 11% of the time after the flop.