You have to know whether villian is on a draw to begin with, which isn't always easy to do, and hard to narrow down on just the flop. It isn't an exact science anyhow, since everything is extremely situational. There's times where betting 3/4 pot is optimal and there's times where betting 1/2 the pot is better. I am actually a little confused about your question so hopefully someone here can help you with the math.