After the initial checkout and calibration I would:
- Rotate the stepper in the "up" direction stopping when the arm contact opens.
- Zero stepper count
- start the stepped down, counting the steps until the arm contact opens again.
- This count will be your diameter base value
- You will have to do some mensuration here to calculate the diameter. Or else you could have calibrated it to some number of known diameters and interpolate.