Having just gone through the process of calibrating the distances on my S3, I think that having CM prompt for a couple of values and then applying the correction would be useful.
It’s not just for belt stretch, either. I have a Z Plus, and it did not move accurately. After calibration, it’s much better. Approaching the measurement point from both directions gave the same value, so I don’t think it’s backlash.
I envision something like the user picks an axis, then CM has the user fill in two values - how much it should have moved, and how much it did move. It then applies the math to the current value in the appropriate $10x register and updates it. Maybe also some sanity checks on the two distances (they shouldn’t be that far apart) to prevent fat-fingering things.