It just occurred to me.... you might not be aware of the very practical reason(s) for variable speed..... because different grits of belts cut more efficiently, and last longer at different speeds, a variable speed grinder can save you money on belts.
Heavy grits, 34-60 grits, work best a top speeds, as grits become finer, speed should be reduced for efficiency, longevity, and heat reduction. Exact settings depend on drive wheels on the grinder, but
roughly.....
24-60 grits FULL SPEED, whatever the drive wheel size.
80-120 grits 50-60% on the speed control dial for a 4" drive wheel (KBAC 2X series VFDs)/30-40% for 6" drive wheel
220-400 grits 40-50% on the speed control dial for a 4" drive wheel (KBAC 2X series VFDs)/ 20-40% for 6" drive wheel
600 and finer grits..... varies based on material being ground and belt type, but I rarely run these grits higher then 25% for either drive wheel size.
Of course,
those are the speeds that work best FOR ME. I'd consider them as BEGINNING points for you, with the exception of what you use for heavy grit belts (24-60 grits) which are almost always best used at highest speed, then adjust to your liking.