Hi Joe,
You aren't being real clear on what you are doing. But I'm guessing you are using the Basic method where no C Programs are required at all. KMotionCNC just scales the RPM the way you specify and commands a DAC as described here:
KMotionCNC Spindle Control
In this case you need to calculate the proper scaling for your system. If -2047 DAC counts runs at 18000RPM then the scale factor to enter would be:
-2047/18000 = -0.11372
But then you say you would like to control the spindle like an axis. What do you mean by that and why?
Regards