There are different sop's I've know of.
Now.most require you to do a continuous descent from the FAF.
On the NG you may select missed app alt when going 300' bellow the FAF altitude, while on the classic is set while approaching the MDA.
One other thing,the point where you reach MDA may not be the point where you initiate the GA,or where you decide to land. There are many procedures where you canmaintain level flight at MDA till the MAP,or where a VP is specified.
We have one particular airport where VP is much ahead the MAP ,and we had some interesting landings,due to pilots going beyond VP to the MAP (which is at the THR) and deciding to land when they saw the rwy,a bit late,you guess...
If you've select missed app alt after leaving FAF is very important to check when arriving at MDA,and pushing alt hold or GA....otherwise...
Brgds...