Minimum defense frequency (MDF) describes the portion of your range with which you must continue when facing a bet in order to remain unexploitable by bluffs. In other words, MDF is the minimum percentage of time you must call (or raise) to prevent your opponent from profiting by always bluffing you.
The formula to calculate MDF is simple:
pot size / (pot size + bet size)
Then, multiply the answer by 100 to express it as a percentage.