Its 30' where I work.
The idea is that, lets say MDA is 400', you add your 50' so that gives you 450'.
You do the approach and at 450' you see nothing.
You start to go around but the a/c is still descending.
As you started the GA 50' above MDA, even though it takes a moment or two to arrest your descent, and you lose a bit of height as you do so, you don't go below MDA.
Whereas if you started the GA
at MDA, you surely would.
Now, back in the day, you'd
level at MDA and then clatter on until you reached the MAP or saw the runway. The so called 'Dive and Drive', so no need for extra height.
Perfectly good, IMHO, but far far too dangerous.
