Instead of taping a ball, we actually color in half the ball (completely fill in one of the two pieces of leather that make up the cover) with a sharpie. With my iPhone, I can count the rotations much easier this way. The challenge is accurately timing how long the ball is in the air, which I do with a stop watch. If you have software that breaks the video down frame-by-frame, you can get a very accurate reading. This also makes seeing the direction of spin easy, too. Here is a slow-mo video of what it looks like.
https://youtu.be/OlIQBZWT6a8
https://youtu.be/OlIQBZWT6a8
