Odds, NLHE, 4th street FD

[x vikram]

NLHE cash game. 10 handed.

Assuming the you arrive against one player with a flush draw on 4th street, the flop was checked etc.

I have a flush draw and am facing a bet of 2:1 however my chance to hit is 4.5:1.

I was doing some simple maths, assuming the pot is 300$ and the bet on 4th street is 300$ you are getting 2:1 and therefore 3 times you will miss costing you 900$ and 1 time you will hit winning you 600$, does this mean for this move to be +ev you must win more than 300$, possibly gained by betting once hitting and being called?

Just wondering how to make FDs profitable from 4th street.