I have wondered too why the pot would be hard to calibrate if you do it mechanically, taking careful note on the old one of arm position vs sensor plate position - but I haven't had the need to try yet. However, according to Bosch, the sensor element is non-linear, i.e. dR/dL is not constant. It is largest near idle, apparently. This would likely make adjustment quite tricky, and it is possible that production variations combined with this might make static geometric calibration useless.