It’s possible you need to calibrate your motor steps…
It was recently discovered that some of the belts that were shipped did not match the exact tooth profile that was expected, and caused some slight inaccuracies over long distances
See here: Houston we have a problem, belts
The generator I believe, already generates it 0.5mm short.
So the file is likely set for 1387.5. If the final measurement is 1385, that sounds within the range of some of the belt error we were seeing