As I said, just set Pulley 1 only, from 0 to 24,000. Make sure Pulley 1 is your current Pulley. Set the PWM min % to 30.
Your available RPM will be only what the Current Pulley is set to.
Then, depending on the commanded speed, Mach3 will output a proportional PWM Signal from 0 to 24,000rpm. By setting the minimum to 30, if you accidentally command 1000rpm, Mach3 will bump it up to 30% of 24000rpm, which is about 7000.
If you've ever tried do calibrate the spindle in Mach3, go to your Mach3\macros\yourprofile\ folder and delete the file "Linearity.dat". Then restart Mach3.