Simple. Mulitply your outs by 4 on the turn, by two on the river. That gives you the percentage of your hand HITTING, not necessarily winning though. Hitting a nut flush is a winner of course, assuming there is no possiblity of 4 of a kind or a straight flush.
You could also have the better hand if you hit a pair if you have overcards. In this case if you also had two over cards, your outs would be 9 plus 2 plus 2, or 13 outs to hit the hand. This has no relationship to betting, only to the odds of winning.
The other idea is the odds of winning the pot verses the amount you need to bet to stay in. I do not bet draws, but will sometimes call with them if I have overcards. I often call a min bet with a nut flush draw and overcards. The question you have to ask is how much is in the pot when you make your decision. If your odds are 1 out of 3 to hit the nut flush, and the pot has $80 in it, and it costs you $20 to call, then you do so. Ir you make the play 3 times, you will win once, lose twice. Since over the three hands at $20 each you would bet $60 and win $80, you would be $20 ahead if the odds hold.
