To answer your question without being as confusing as all the previous posts...

(Your Call)/(Your Call + His Bet + Pot before his bet) must be less than your percent chance of hitting your hand.


The real life situation is more complicated, because theres still more betting to come after you make that calculation and decide whether to call or not...If you make your hand on the turn will you be able to make him pay you? This essentially increases the denominator of the calculation by adding another number "+ How much more you think you can get put into the pot" . Thats implied odds.