It is difficult to calculate the optimum speed in theory. The ideal rotor speed depends on many factors, especially viscosities, evaporator / rotor design, wiper type, ratio residue / distillate, load and others. Best way is to ask the manufacturer for the design speed and start from there with trials. Change the speed and analyse your product properties.
A proper online measurement (in your case distillate purity) can be very useful. As a change of rotor speed takes almost immediate effect and the residence time in the evaporator is usually short it is easy to do a few tests within short time.
In the past, UIC GmbH has organised training courses on the basics of short-path and thin-film evaporators. They may have further documentation. I can also recommend operating a small lab unit made out of glass. You will see a lot when the wiper moves through the liquid.