Reverse pendulum serve contact height

Hello, 

Thanks a lot for answering my previous questions, your suggestions were really helpful. I have tried the reverse pendulum serve several times and I have been successful a few times. But while practicing, I want to make it into a routine. So could you give me a convenient reference point (with respect to the body) where the contact should start. I mean should it contact happen when the ball has reached just below the chest or at the stomach or lower. Many times, I am indecisive about the height at which I should contact it. So I would be very grateful if you shall give a reference point so that I can practice with it and perfect it by always contacting the ball at the same location.