20% would be fine, but in my case it looks more like this. (guess it could be the VFD in the speed control as suggested)
Call for -> Get %diff
300 -> 163 -46%
400 -> 281 -30%
500 -> 396 -21%
1000-> 989 -1%
1500-> 1563 +4%
2000-> 2155 +8%
Normally I mill around 2500 which is the top end of my spindle speed (and that is all totally fine), but in this case, I needed to run a face mill at around 350 rpm. It came in under-speed, with a stall and a crash
Of course, I can work around this by jacking up the requested speed to ~800 in CAM, or by adjusting SRO slider on Mach4 (which is what I did). However, both of those methods introduce other potentials for human error on a future date, so if there was a way to fix it, I would like to save future self the headache =)
I am using a Vital systems HiCON Controller (and set the Analog Spindle Scale to 48.8 in the mach plugin to get it 'close'). That output feeds into a Precision Matthews PM-25 stock speed control. I did measure the output of the HiCon card to be generating expected voltages. From there wired into the location where the analog Speed Potentiometer on the PM-25 would have attached. (The rest of the board is potted with black epoxy, so I have no idea what is in there, or able to test further)
Thanks for the info!