Not sure what common has to do with the XP per turn in.
I got about 78-79% of a level from 15 belts. That is 5.2% per belt. P99's own wiki has level 6's receiving 10-11% per belt. I'm a high elf enchanter with a 10% penalty. Does not compute.
https://wiki.project1999.com/Crushbone_Belts