The Celestron focus motor does 1000 steps per turn of the focuser knob. I don’t know what that means as change of focus position because it depends on your scope. SCTs are tricky because moving the primary mirror alters the focal ratio of the scope.
If you really want to know the movement of the focal plane per motor step then this should do it:
- Focus normally. Note the steps.
- Add a spacer to your camera to move it out by a small and known amount, about 5 to 10 mm but whatever you can manage.
- Focus again, note the new position.
- You can now calculate the step size from the space thickness and the difference in focus position in number of steps.
Personally I wouldn’t bother, I’d start by setting 200 steps and trying it. If the movement is too small double it, if too large halve it.