Fred,
Position the axis so you can make a move as long as possible .....a move you can measure. Remove any backlash, you move one way and then a small distance the other way.
You will asked how far to move, lets say 10", the axis will move 10" based on the current steps per unit settings, it will then will ask you how far you
moved....you measure and find it to be 9.950" and that's what you input for the value. The steps per will be recalculated and you will be asked if you want to change them, you reply ok, if you go to motor tuning you will find that the steps per have been changed. Save settings if you want to keep the revised settings.
You can use the mill screen for the tuning, but, make note of the values and use them in the turn screen.
Once you have the steps per correct then you want to try a a few intermediate steps and see how linear the axis is in movement.
With the steps per unit correct then you want to check for backlash.
RICH