Instrument approaches contain an apparent level segment, so for an RNAV, the MDA starts (usually, unless there is a step) at the final approach fix. I recall being taught to descend to the MDA as soon as possible after the final approach fix, then fly straight and level if not visual, in order to maximise the chance of becoming visual and then being able to drop the nose and descend to the runway. However, at the airline I fly for, the RNAV is a constant descent approach to the MDA. The missed approach, including an immediate climb of course, is conducted from the MDA, and not the MAPt. There is no level segment. In theory, were I to be 500ft low for the entire approach to the MDA the missed approach may well occur several miles before the MAPt is reached. The MAPt is largely irrelevant, in this case, and passes un-noticed.