From memory so hope the following is correct,
You put a desired moved value in for the axis, Mach reports back and says here how far it moved, if you agree with reported move distance , click yes, then it will ask if you want to save the steps per unit and will change the steps per unit in motor tuning. If the moved value is not what you input, then put that value in and try again. Very quickly Mach will find the steps per unit value which is correct for the movement you wanted.
RICH