I haven’t been on the forums for a while and wanted to offer this idea for belt calibration.
After running it on a ruler to get it fairly close first. A larger measuring device would be good. I use a v bit with a flat spot but anything will work I think.
Set up the digital calipers parallel to the axis you are adjusting. Use three blocks that are the same height. Use a tie down to hold the body of the caliper. Make sure it can move and zero it out against the bit. Move and read the measurement, adjust etc. I usually do each Y axis separately but it seems to work pretty well with out guessing on tension.