Not BMR + exercise; that's too low. Probably the third mfp way. The only problem I see is that even sedentary has some expectation of walking so a few calories could get double counted. Most people in maintenance on mfp (or at least the ones discussing it) tweak the calories by hand instead of relying on mfp's calculation. They start with that third way and monitor weight and adjust the base up or down to compensate for losses or gains.