-The answer is given by analyzing the size of the bet in relation to the pot and the probability that the hand they wish to exit at the next card. For example, the pot could be $ 40 on the flop and someone already made a bet of $ 40 and is in the same flush draw. The pot is now $ 80 and see the next card will need to call, this bet will be profitable? We already know beforehand that that the probability of a heart on the turn is approximately 20%, then just do the calculations on which on which the expected profit on this call:

($ 80 (pot) + $ 40 (call) * .20 = $ 24

-As you can see its expected profit is much lower than the bet you just made and hence the long term will lose money if you make this move.

-So when a bet will be profitable so that we call the flush draw? The equation: (80 + x) * x = 0.20 (80 + x) * x = 0.20

-By solving the equation verifies that X is 20, so the maximum bet that can make call without losing money is $ 20 in other words should not to call a bet greater than $ 20 in this situation.

-Having said all this should always take into account all these factors when playing they are all equally important and none should be overlooked.