The ratio is simply to set the ratio for Index to spindle if they are different. The reason for that is it is often not possible or difficult  to place your Index on the spindle itself.
Also make sure your max spindle speed is set to what your max speed actually is, if not then you will not get accurate output from Mach. For example if your VFD is set up so that 10v represents 2000rpm and you have 4000rpm set as the Max in spindle pulleys then when you command 2000rpm you will only get 1000rpm at the motor because Mach thinks 4000rpm is 10v.

Hood