Anthony, I come up with the belt speeds from calculations. When I was using the step pulley setup, I could take the fixed motor RPM, and the pulley ratio to calculate the belt speed. With the VFD, I programmed the VFD to display the belt speed based on motor RPM using the VFD algorithm. VFD drives will display motor RPM based on internal calculations and percentage of speed called for. I entered a factor to give actual belt speed. If you have a a VFD drive, don't worry abut running less than 20% or so, especially for fairly short periods of time (5 to 15 minutes). If you're concerned about motor overheating, just lay your hand on motor and you can tell if motor is getting hot.
Going VERY slow isn't going to hurt bearings, belt, etc at all.
Ken H>