My sizes are out

[Neale]

Sounds like your "steps per unit" calculations have gone a bit wrong. Can you recheck, or post all relevant data here so someone else can check your working?

The other way of doing it is starting with a good first guess - which you have - and then modifying the "steps per" based on the results of a calibration run. Personally, if my calculated value were out by more than a tiny amount then I reckon something's wrong - my calculation or a machine error. Slipping coupling, missing steps, or whatever. However, others take the more empirical approach. In short - keep tweaking until you get the right answer! Mach3 actually has a calibration capability built-in but don't know about your controller.