A low topspin ball is a double-edged sword. It is a good putaway shot but if it lands short it becomes a perfect setup ball for the opponent. I think you should hit your topspin relatively high above the net so that it will land deep and bounce high keeping your opponent at bay. However if you hit it too high you would be labeled a pusher, a moonballer or something worse, so where should you draw the line? 4ft, 6ft or 8ft?