I know the set your talking about, I just gave one away, it was an older model I had laying around. Nice machine to work with. When I used sandpaper from left to right my wheels were 80, 100, 220, 400, 600. On the far right end wheel I could just slide the sandpaper belt off and would always swap out with a 3000 grit polish and then a micro polish and then I could screw on the buffer wheel there and do the polish.
I dont think that speed should be a factor as long as you have clean water running on the wheels, does the water pump up from the tray or does it drip/spray down on the wheels? Also depends on your technique and amount of time spent on each grit. check out the RTH forum website I put on the post and there are guys out there who have old manuals for all these machines and could give you the ratio your looking for. Alot of good info for questions there. When you ever switch over to nova wheels you`ll be very happy, I was.