The famous 50 ft
411A, if you want to fly level at MDA, how high above MDA do you have to start levelling off, in order not to sag through it, would about 50 ft be a nice estimate?
In that way the margin discussion turns into a storm in a bowl of dust.
The 50 ft have just become a sort of industry standard figure for most types (not all, e.g. the famous MD-11 does not require that margin, lightning fast acceleration on Go Around apparently)
Am not gonna chastise you for the D/D technique, it's just that the trend in regulation and in many outfits is moving away from it and today's avionics make it possible to achieve good results with the "artificial glidepath" technique, an accurate present position is the basic element that makes it possible (yes, some aircraft types had that already eons ago).