IIRC- belt rank is based off how much QI you have compared to how much QI others have with the same belt. If there is a tie, it's decided by W/L
Additionally want to clear up confusion on last belts:
I'm positive god belt is 50k, then I think one belt is 100k, and elite belt 1kk. I remember hearing QI resets at 14kk but can't confirm