Kevin don't know If your aware but Mach has a auto step setting feature that is very usefull for fine tuning the steps per.!

To use it you need some form of accurate measuring device, A tape measure hisn't good enough.!! The easist and best way I've found is to use a digital dial gauge.
Fasten the dial gauge to the moving portion of the machine IE Gantry or table if mill but make sure the slide on gauge can still move. Then Open the dial gauge fully and zero, on the next step you will set a distance to travel just less than the dial gauge travel. The gantry or the table will push the slide against the spindle or a stop.

On the settings tab there a button labeled " set steps per unit" just above the reset button, click then select Axis to setup. In the next box enter the distance you want to move, when finished enter the amount shown on dial gauge into the on screen box.! . . Now your axis is set accurately.
The longer the distance you can measure over the more accurate your steps per will be.
I suggest for the first try you enter a small value, if ok which it should be then go again with the max distance you can measure.