Read your "Magic" answer slowly. You're going to go down the GS till you reach DA, then you're going to set your altimeter to read DA? Why, you just read DA off it? You don't have a reference point. Give me an old QFE altimeter and I can set a 200 MSL DH anywhere from below sea level to at least 8000 MSL. Are you talking about reading DH off a radio altimeter and then setting the baro to read the matching DA? Not sure what you're flying but even slowed to 100 knots your vertical speed will be 500 fpm, that over 8 feet per second. Which is about .01 change per second. When you saw the RA at 200 and then looked at the baro and set 200 feet you wouldn't be at 200 feet anymore.