Just to be an echo of what everyone else has said...MDA is a hard altitude. For an instrument ride or IPC it's +100/-0 feet on an MDA. As for the DH/DA, that's when you make the decision to go around. If you're making this decision at the DA, then you most likely still in the soup. Since you can't see, you want to make the maneuver somewhat expeditious, yet smooth at the same time. I agree, if you lose more than 20 feet past the DA in a single-engine cessna, then you've done something wrong. If you're in a 747, that's a different story...It's all relative.