Check out the stepper motor calculator in this thread: Stepper Motor Calibration Calculator
It uses the distance traveled, the distance it SHOULD have traveled, and the current stepper motor steps/mm. If you have those three numbers for it, it will tell you what steps/mm you should change to. To convert from steps/mm to steps/in, just multiply the steps/in by 25.4 before you plug it into the calculator, and divide the result it gives you by 25.4.