*Originally posted by Tosh5457*

I bet $0.25, so I only need to take the pot 1 in 2 times to be EV=0 (lets say there is no rake)...

..assuming you never win if called.

Using your example and assuming that your flop bet puts you AI with villain covering.

EV{semi-bluff}= FE*pot +(1-FE)*[pot_equity*potsize_when_called - bet_size]

=FE*0.25 +(1-FE)*[36%*0.75 - 0.25]

where FE = Probability of you taking the pot straight down and your pot-equity is assumed to be 36%

In real life though you have implied/reverse implied odds to worry about besides villain's hand-range and general playing style.