% to pot odds conversion:

(100 - %_to_hit)/(%_to_hit) = odds to hit.

As long as those odds are greater than the odds the pot is laying you are 'correct' to call.

In your flush example, you say you have a 36% chance of hitting (and assumed winning). So, your pot odds are (100 - 36)/(36) = 64/36 ~= 2:1. The pot lays 4:1 to you, which is greater than 2:1.

Of course, you are 36% to hit on EITHER the turn or the river when drawing on a 4 flush on the flop. You need to evaluate your odds based on hitting on the next card because a bet may be coming on the turn which becomes its own pot odds problem.