basically the motor has 200 steps per revolution, if the pitch of the ballscrew was 1mm and the screw was directly driven by the stepper, then one revoultion of the stepper would give you 1mm set at 200 steps. But if the pitch was 2mm then it would have to be set at 100 steps to give you 1mm movement. It gets a bit more complicated when the stepper drives the ballscrew through a toothed belt and two different sized pulleys, as in the boxford. So to get back to the 1mm movement for one revolution on a 2mm ballscrew you would have to gear the motor down by 2:1 at 200 steps.