I didn't say 50ft. It is 1/10th of the vertical speed at that moment which is at least 70ft to 100ft before MDA. It is sort of manual level off. Airbus prefers to do it this way and it works. However since you are going to fly level at that altitude it could also be done by setting that at FCU and allowing the aircraft to level off on its own. May be Airbus doesn't want to have a different procedure just for one type of approach.