For me, my shots tend to go about 1 foot over the net...maybe. I play very flat, especially when I can step into the court. I do possess more rally type balls with the ball going about 2-3 feet over the net.
I really do not think visualizing height over the net is very important. We can of course use this as a determining factor to see how much spin we're getting, and stringing a cord a few feet above the net during practice will give us instant feedback on whether or not our strokes are getting good clearance.
Yonex RDiS 100 Midplus, Tourna Poly Big Hitter Silver 17 @ 54lbs.