Steps per unit is the number of steps required for 1 unit (mm) of motion, for common belt movement, no steps per revolution / (pulley diameter * pi) wil give this figure. If you're full stepping with a 200 step motor, using an 18mm pulley:
200 / (18mm * pi)
200 / 56.5487mm
= 3.53
If you're microstepping at 16 micro-steps per step, then multiply the number of steps in your stepper by the number of microsteps, and feed that into the equation:
3200 / 56.5487 = 56.588 steps/mm (steps/unit)
200 / (18mm * pi)
200 / 56.5487mm
= 3.53
If you're microstepping at 16 micro-steps per step, then multiply the number of steps in your stepper by the number of microsteps, and feed that into the equation:
3200 / 56.5487 = 56.588 steps/mm (steps/unit)