There is a lot of technical info behind the measuring procedure that I don't want to get in to, but basically what you need to know is: You just need one bounce one the center of the blade. The ball height just needs to be enough to get a clear reading. Ball height won't affect the the peak, any small changes are due to microphone sensitivity. Grip pressure and position will affect the reading because you are changing the damping properties with your hand. What I normally do is to lay the phone on the table and hold the blade near the microphone with my normal playing grip. Then I drop the ball from 30-40cm and get the reading.