DJH.
I have a Taig here and will measure the pulleys later and work out what the various spindle speeds are.
Can I query your motor being 4,000 rpm ? I was led to understand that the Italian motors supplied with a Taig were 2,850 rpm ?

Ratio's are just a division of driver over driven multiplied by motor speed so a 100mm motor pulley driving a 50mm spindle pulley will be 100 / 50 = 2 x motor speed, if it is a 2,850 motor then spindle will be doing 5,700

John S.