The recommended height varies quite a bit on the TopspinPro.

For adults learning, we typically suggest the ball is around a hand width below the waist so that at contact, the racket is roughly horizontal. To make it easier, visualise a clock face with the racket as the “hands”. To get the hand-action correct for topspin, we advise that you should focus on a small part of the swing that is pre & post-contact. We typically try to set it up around waist height for everybody. Having said that, for children, the ball tends to bounce higher relative to their body height than for us adults.  So it might be more realistic to set it higher.

Und wenn die Leute erst einmal mit einer bestimmten Höhe vertraut sind, variieren wir die Höhe gerne ein wenig, damit sie ein Gefühl dafür bekommen, wie sie fast identische Handgriffe in verschiedenen Höhen ausführen können.