Where are the belts from? We’ve observed multiple instances of belt teeth spacing differ enough to be measurable over 8ft lengths. Even when cut from the same self sourced cheapo belt spool.
But even then, variance has only been 1-2mm or so for ~8’ builds like yours. So, a 10mm variance is shockingly large (and an interesting number…), unless you share these were discounted close-out sale from temu or something?
Consider ordering belts from Ryan, or a supplier he recommends. Ryan’s tested and vetted and few different belt manufacturers now.
Related belt topic https://forum.v1e.com/t/houston-we-have-a-problem-belts/46656 chasing down ~0.1% variance. Which is very different to the 1% variance you’re seeing, so that topic may be unrelated.