Thanks for the link!
I’ll assume that given a room rarity, each room has equal probability. If so, then using the above link, the average chance of a chest upgrade from an epic room is (45% + 6*40%) / 7 = 41%.
Faction rooms before the boss:
All-Seeing Eye ---- entry, 1 leg, 1 epic, 1 u. rare, 2 rare, 1 common
Crypt Keepers ----- entry, 1 leg, 2 epic, 2 u. rare, 1 rare, 1 common
Hall of Guardians - entry, 0 leg, 3 epic, 2 u. rare, 2 rare, 0 common
Adding the chances, it seems that for Hall of Guardians the average chest level just before visiting the boss is 1 + (3 x 0.41) + (2 x 0.28) + (2 x 0.19) = 3.2.
Average chest level before defeating the boss:
All-Seeing Eye ~ 2.65
Crypt Keepers ~ 3.15
Hall of Guardians ~ 3.15
With less “rounding” 2.652, 3.146, 3.162. (I don’t want to list crypt vs hall as 3.1 vs 3.2 so cheated the rounding a bit. If room chances are unequal, their order might switch.)
If anyone else tries to calculate average chest level, let us know if your answer is the same or different.