question on how often my opponent needs to fold to make a semi bluff +ev

[grumpy64]

i know i could use a FE calculator but i think those only work when shoving.

Ok I have A FD on the turn and for the sake of simplicity lets say i can never lose if i river my flush. lets also say i'm gonna shove river every time i hit and V will call a river shove 20 percent of the time. my bluff is 3/4s pot and effective stacks are 100xbb and so far pot has 10xbb in the middle. so our bluff is 7.5 thus we will lose 7.5 81 percent of the time and we will win 100 0.038 percent of the time so we lose 7.5 when me miss and win 3.8 when called. if he never fold we lose -2.275.

(100*0.038)-(7.5*.81) I just don't know how to figure how often V needs to fold in this situation to make the bluff 0ev