The recommended height varies quite a bit. 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 to visualize a clock-face with the racket as the “hands”. To get the hand-action correct for topspin I 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.

And in fact, once people are comfortable with it at a certain height, we like to vary the height a bit so they get a feel for using almost identical hand actions at several different heights.