You can do a ton of math trying to figure the right size capacitor and still have to use some trial and error. The easiest method I have found is have a capacitor of the proper voltage rating(at least line voltage) connected to the motor.
(1) Make available both capacitor leads and both sides of the start winding, then apply power to the motor with a locked rotor.
(2) Using two volt meters, with meter #1 read the voltage across the start winding and with meter #2 read the voltage across the capacitor.
The voltage across the capacitor should be 10% higher than the voltage across the start winding. When this is achieved, you have the right capacitor.
The reason you use two volt meters is that this test should be short in duration, as quick as possible not only for safety but as the readings will change as the windings heat up.
A word of caution, capacitors can explode when line voltage is applied therefore cover the capacitor with a cardboard box or something.
Roger