I am currently using Mach3 in PWM mode to a newforever E100 VFD for spindle control.  I was not able to get closed loop control tuned for sufficient response and accuracy so I am running it open loop.  In this mode, it is responsive and repeatable.  However, I can't set the speed real close as the PWM set point resolution is a function of the clock frequency.  In addition, the accuracy is not spot on and there is a bit of noise or jitter in the PWM output as you might expect.  An example after doing the spindle calibration CFG and then I enter a setpoint for 2000 RPM I get ~2080 RPM with +/- 5 noise.  I recognize the noise is insignificant.  But I would like the set point to be closer.

Other than running the auto spindle calibration, I don't see where I can set the scaling for the PWM output.  Can anybody help with this?

I am considering setting it up for MODBUS control.  From some of the posts I came across, that looks like a bit of a challenge.  If you make it through that challenge and get the MODBUS functioning, is it more accurate and stable than PWM?

Unless your VFD has an encoder on the motor shaft or is a sensorless vector control type that can determine shaft position using back EMF then you won't get a really steady speed in any event.  Unloaded the spindle will be nearly synchronous with the Hz output. When the tool touches the work or the depth of cut changes the speed will go up and down by 60 rpm or so.  So I'd not be too worried about it.