To find a minimum speed the smallest pulley would be on the motor.
With a 4" pulley on the headstock and a 2" pulley on the motor the ratio is 2:1
A normal motor of 1725 divided by 2 would be approx 862.5.
Your theorie suggests a high speed setting with the large pulley on the motor.

Or: Measure your pulley sizes - outside OK, since you're interested in a
ratio. If the Pulley on the motor is 4", lathe 2", the ratio is 2:1.
Multiply by the motor speed.

If I were a betting man, I'd say minimum is 600.


Bob, Naugatuck Ct.
http://www.outofcontrol-woodturning.com