I was one of the first to receive a Prusa XL Multitool printer. From the start, I had problems tightening the belt equally on both sides and still aligned parallel so that there is no gap on either side.
Since I've spent a lot of time researching the problem, I know many people have this problem. So I tried a lot of things and have finally found a working solution!
After each adjustment, the entire carriage must be rotated 1-2 times completely back and forth and completely left to right and back to the belt to evenly distribute the tension across the entire belt. Only then measure again. (Video)
The adjustment position is always in the very center of the front.