Ah, cheers Jarulf, based on area level not progress through the act, that makes a bit of sense.
Cujo, the TreasureClassEx.txt file simply lists each of these Chest TCs and has an Equip TC alongside it that it can drop from. You can't "work it out" as such, you have to look it up. But I suppose I could list the Hell ones here...
Format is "Chest TC" "Equip TC" "max item TC level". Apologies if it is hard to read, never worked out posting tables properly...
Act 1 (H) Chest A Act 1 (H) Equip A 63
Act 1 (H) Chest B Act 1 (H) Equip B 66
Act 1 (H) Chest C Act 1 (H) Equip B 66
Act 2 (H) Chest A Act 2 (H) Equip A 69
Act 2 (H) Chest B Act 2 (H) Equip B 69
Act 2 (H) Chest C Act 2 (H) Equip B 69
Act 3 (H) Chest A Act 3 (H) Equip A 72
Act 3 (H) Chest B Act 3 (H) Equip B 75
Act 3 (H) Chest C Act 3 (H) Equip B 75
Act 4 (H) Chest A Act 4 (H) Equip A 78
Act 4 (H) Chest B Act 4 (H) Equip B 78
Act 4 (H) Chest C Act 4 (H) Equip B 78
Act 5 (H) Chest A Act 5 (H) Equip A 84
Act 5 (H) Chest B Act 5 (H) Equip B 87
Act 5 (H) Chest C Act 5 (H) Equip B 87
A lot of those look the same, there is in fact no difference between most Chest B and C TCs.