Maybe you guys have already answered this question but how much does it really move after power off and then on? If you power off with tension on it knows the belt lengths and retraction force then you power back on use the same known belt length give it the same retraction force and whatever the difference is between the old and new length values would become an offset applied at that time. Subject of course to change each time you power off/on. I’m not a programmer but the basic steps in my mind are
Belts under tension with know retraction force
Power off belt lengths saved
Power on saved belt lengths loaded
Machine uses known retraction force before power down to tension belts and read a new belt length
Differential between old length and new length supply offset to apply to new belt lengths so they are the same as before power down.