Alright, I know it varies, but I need a general number.

My 10x22 lathe has a six preset speeds, the minimum is 150 RPM, and I'm wanting to avoid tinkering with the electric bits. I may decide to go that route, but I think if I instead gear it down (using the tensioner as a place for new gears) through two short belts instead of one long one, I could cut it down to, say, 50.

I can never remember--is it diameter or circumference of a pulley that affects the ratio?