First of all you need to find your current $100 value, open Easel -> Machine Inspector and in the Console window type:
$$ then hit Enter
A $-list will appear, locate the $100-value. Say it says =40.
Recalibrate step/mm by, assuming you get 252mm travel for a 250mm command:
Current $-value * Commanded travel / Actual travel
40 * 250 / 252 = 39.68
Insert new value by typing in Console:
$100=39.68 followed by hitting Enter
Restart controller to make change take effect. Re-test 