The fold rate a bluff needs to break even when the bet is ⅓, ½, ⅔ or pot.
The formula is alpha = bet ÷ (bet + pot). Betting 5 into 10 needs 5 ÷ 15 = 33%. If they fold once in 3, you break even. Bluff opponents who fold more than that; don't bluff those who don't. Soft live opponents are almost all players who don't fold.