Alpha
Alpha is the fraction of the time a bluff must work to break even, calculated as bet divided by bet plus pot. It is the complement of the defender's minimum defence frequency.
alpha = bet / (bet + pot)
Alpha answers the bluffer's side of the MDF question: how often does the fold need to come for a zero-equity bluff to break even?
Worked example. The pot is 100bb and you bluff 50bb:
alpha = 50 / (50 + 100) = 0.333
The bluff needs a fold one time in three. If your opponent folds more often than that, the bluff profits before your hand ever matters.
Alpha and MDF always sum to 1 for the same bet size: alpha is the attacker's break-even fold frequency, MDF is the defender's answer to it. Bigger bets raise alpha (the bluff needs to work more often) and lower the defender's required continuing share.
Try it live
The MDF calculator below is the same tool as the standalone page, running right here.
Pot before the bet, and the bet villain makes into it.