I haven't gotten my X-Carve yet but I have been tuning the stepper drivers on my 3D printer.
FYI: 3d printers do not put as much load on the motors so the workable range is a lot bigger.
Too little current and the motor will slip. It usually makes a distinctive "clunk" when this happens.
(To see what this is like you can dial back the current while the motor is holding until you can force it to turn by hand. You can feel the "slip" and hear the 'thunk".)
Too much current and the driver chip will over heat and go into thermal shutdown.
(From what I understand this often causes a noticeable "stutter / pause" in the motor sounds. I have not heard it but considering how the steppers "sing" I suspect any kind of pulse pause would be audible.)
To do it "right" you need to hook up your meter in series with the motor (wire into one meter wire, other meter wire out to the motor) to measure the current. Kind of a pain and even then not "perfect" as things will change under load.
A lot of people literally adjust it by ear. Running the motors and listing the point where is starts to sound "bad" (this has something to do with vibration and harmonics?) and backing off.
I did it by temperature.
I was trying to rule out thermal shutdown I didn't have a good way to measure the temperature of the chip so first I measured the running temp of the stepper motor. More current means hotter motor. (The motor I have has a max operating temp of around 90c before it fries so I wanted to stay well below that.) I taped a digital cooking probe thermometer to it with some foil tape and monitored its temp. (I called up a 3D printer company and asked them what temperature their steppers were running at. A really weird question but easy to check, they measured them and found they were running around 45c. I ran mine from 40c to 60c finding that 45c was a good minimum for my application as well.)
I was still having issues so I bought a thermocouple probe thermometer and used some kapton tape to tape it to the actual driver chip. This was a pain. I put down a layer of tape so the probe leads would not short anything, then more tape to hold it in place. The chip has a max operating temp of 85c then thermal shutdown kicks in. I found that my chip was running at 55c at peak load. Damm "hot" to the touch but well within it operation range.
So what does this all mean for the x-carve? I am not sure. But maybe we can get the running temperature of the steppers of a well tuned machine to compare against?
What do you think Inventables? Want to tape a probe thermometer to a stepper running a 6 hour print and see what its running temp is?
I know it is not a very direct way to gauge stepper current. But it is a fairly easy thing to do (Compared to directly measuring the current) And might be a way to get a feel for if your current is "Too High" or "Too Low".
Edit: Of course stepper current is not the only source of missed steps (it was not with my 3d printer either) You need to make sure you belts are not slipping.