Adding a rotary axis for tube notching

I’ve been thinking about this one for a while myself. I have been reading the thread and can’t get my head around why in this set up different size pipe would require different settings for the stepper. Isn’t that handled in the drawing of the cutline? If the stepper is turning at a constant mm per revolution then the variable is pipe circumference. if you use a software to figure the cutline like metalgeek.com you put the pipe diameter in there and it adjusts the length of the cut to suit. In another thread here @djlois shows how to reset the number of steps the controller thinks it takes the motor to move a given distance. To me it seems like that is the answer to the pipe diameter issue? What am I missing?