What I meant was set rpm/v to something like 350 or 400 rpm/v so there is plenty headroom, just to eliminate the drive not seeing the full 10 volts. Due to various factors, the drive might never see the full 10V, so if you set a higher rpm/V, then the drive should max out long before 10V is needed.

What you are saying is roughly correct.
Mach requests 3000rpm from the controller, which then increases the voltage to achieve that speed, however the speed maxes out at 2990rpm despite the controller outputting maximum voltage, meaning you then have an ever increasing following error of 10rpm.


Have you checked what speed the drive thinks the motor is doing?
You can normally configure the drive to show motor speed on the display, and if you have a discrepancy between the drive and the controller, then there could be a totally separate issue.


Regarding allowable following error. As long as you're not doing positional work, then I wouldn't be too concerned. Being a couple revolutions away from commanded destination isn't likely to cause any issues when turning (thread cutting would normally work of actual position, not destination so the lag shouldn't cause any problems), however I would expect to be able to get position within a few hundred counts reliably.