Things may have changed, but back in .40 for sure, if you dropped troops below about 400ft, they don't survive. I once lost a capture by dropping at 200ft because of this. However, I have heard of, and performed a successful drop at about 5 ft off the ground.
So the answer to your question is 400ft minimum for parachute drop IMO. Also, your target is the little dome by the tower (map room), drop as close as possible to it. My method usually consists of a run at 20 ft off the ground until right over the base. I then pull up to around 450 ft and try to fly a spiral around the map room while dropping the troops.
There is one more method I've used successfully that is very tough to counter. Land your c47 and taxi over to the map room, then begin releasing your troops. As long as troops can be deployed while on the ground, this is the fastest way.
If a good quality vulch-fest is taking place, this is usually the best method IMO.
These are the two ways that I prefer to drop, but I'm sure there are others.
Hope this helps you.
[This message has been edited by bloom25 (edited 01-24-2000).]