In traditional tennis coaching, players are advised to make contact with the serve as high as they can reach.
But if you wish to create topspin on your service, then this High contact point is impossible.
Topspin cannot be achieved without a low to High movement, so therefore the contact with the ball has to be lower than the contact point for the flat or slice services.
We advise that after the “scratch back” racket drop, you try and make your contact as low as possible! It is very unusual if the racket drop is achieved that contact can be too low, but if it is, then this is simple to rectify.