hi

My understanding is that if the price of calling vs pot size is below the pot odds for your outs, then its +ev. is this correct? and do you use the pot value before or after your (potential) call?

If it is correct, then there's something else im unclear about:

You are on say a flush draw from the flop with odds of about 34% and villain does a bet which is +ev for you, you call, you dont hit on the turn. Now the villain bets again, and your pot odds are about halved and the call is -ev. How can the first call have been +ev at 34% if its only effective for the turn?

thanks