In the flight crew techniques manual Airbus says the level off shall not be lower than MDA(H) by pressing the ALT pb. You are in flap 3 gear down, possibly doing over 1000 fpm with a tailwind, the technique adopted by some is to add 50' to the MDA so when you press alt at the programmed MDA you will not bust the charted MDA.
They recommend 3"/100 ft above the runway abeam the threshold, but then go on to say that is for ICAO PAN-OPS and JAR OPS. For FAA or Japan TERPS refer to company SOP.