It could be possible for a faction with few rooms but mostly legendary rooms to have a better upgrade chance than another faction with more rooms but mostly common.
But between Hall and Eye, I think you’re right. Here’s my attempt to calculate the chances:
Do those numbers feel right? At quality 0 that would be 4.2 and 3.6 after defeating the boss, or 7.2 and 6.6 at quality 10.