re: Poker & Extract value when on a NUT flush draw?
Bet as if you hit your A. Not as if you are on a draw. Now, I am assuming you would not go all-in if you had had an A. I am assuming that your bet, if you hit, would be close to pot sized. A small bet looks like what it is -- a value bet on your semi-bluff. Or a c-bet on a complete miss. However, if you normally make a small bet when you hit, make that small bet.
A pot-sized bet is more likely to make the potential check-raiser simply call -- do you have 2 pair to his AK? What's going on? He might give you the chance to catch your spade cheaply by checking to you on the turn and you can get a free card then.
Obviously, if he shoves back at you, you have a decision to make. It's not an insta-fold when you have the nut-flush draw. At least, it's not for most people. I'd stick with it more than half the time.