LetYouDown's odds are dead on, of course, but he leaves out one aspect of your reasoning that needs revision: the "rule of four" to which you're referring calculates odds on hitting with the turn and the river to come . These odds are only directly applicable if you're going all-in, or you doubt very highly that you'll be facing a bet on the turn.

More often, if you're drawing then you'll need to call a bet on the flop and the turn. This can skew your calculations considerably if you're drawing using odds for the turn and river. For instance, if there's $6 in the pot to your flopped flush draw, and your opponent has bet $4, you'll conclude that you're getting 5:2 on your call. With a 35% chance of hitting a flush draw by the river, you would be correct to call if the $4 put you all-in.

But if you're not all-in, and you don't hit your flush on the turn, you'll almost certainly be faced with another bet. With $14 in the pot, if your opponent bet $14, calling for the flush draw could not be right given the 2:1 pot odds. Your call on the turn was incorrect because on the bet you were making - the $4 on the flop - you had a 19% chance of hitting one of your nine outs. The 5:2 pot odds render this a poor call.

That's why unless you're all-in, use the odds of hitting your hand on the turn if you're calling on the flop. The odds of hitting your outs on the turn are almost the same as hitting on the river, so the "rule of 2" is a decent starting point. Using the rule of 4 every time on the flop is a sure way to overvalue your draws.