Very interesting. That accords with my observation. However, I thought that it might be even higher on clay. So if we take this as accurate, Nole's net clearance on average was under 2 feet on clay. Nadal's average was between 3 and 4 feet over the net.
Watching the pros in person, you see that the preferred shot is hard, fast, and low with a lot of topspin, especially on the forehand side. The backhands tend to be hit flatter.
With the computerized shot tracking it should be possible to get extremely accurate readings. IBM used to have shot-tracker(?) at the US Open which would show the complete ball path for each shot.