I have an exhaust fan that operates at 859 rpm with a 1725 rpm motor. The fan sheave has a diameter of 9.75" and the motor sheave has a diameter of 4.625", distance between shafts is 20.75" but can be adjusted ~+/- 2". I am switching the motor out to a 3450 rpm motor to attempt to have the fan achieve around 1200 rpm. What are the formulas, or where can I find them, for determining appropriate fan/motor sheave sizes?