I came across a reference that the shuttle’s braking rockets slow it down 349 feet/sec, or roughly almost 240 mph. Presumably, if it’s going one foot/sec below orbital velocity, it will start dropping, but not quickly enough. Is 349 feet/sec pretty standard? Considering that it’s going over 17,000 mph, that seems a rather small amount to make it start dropping quickly enough to plan re-entry.
They need time to spin the shuttle around after firing the rockets. Also, the rate of descent determines how hot it will get. Fall too fast and the shuttle is guaranteed to burn and break up. Fall too slowly and the shuttle could bounce back up, like a rock skipping on water.
I’m not sure what you mean when you say “plan re-entry.” It’s all caculated ahead of time. As it is, they seem to travel about 10,000 miles before entering the atmosphere, and then travel another 5,000 miles through the atmosphere before landing.