I've always used 220 as it seems to get it done but not leave any snaggy bits which could cause a cut or tear. As a woodworker with a bad habit of using my hand to stop the sander I can say 80 and 120 leave a pretty rough surface on the skin.
I buy a foam sanding block that has two grits on it. The rough is probably 100-120 and fine is 180-150. Go to your local Home Depot and try before you buy. I think they’re 3M. Purple.
I bought the dremel attachment things for my chuck drill, they seem to work good. Probably around 120 grit? At the crag I use the little foam blocks, they’re cheap and have a few different grit options. In combination with straight edge nail clippers they work great for removing skin to minimize splits.