Prob based on D&D, meaning:
- Level (CR)
- Monster type (HIgh Ref Evasion archers etc)
But you should also consider that:
- Reaper inscreases their saves, speculated a -7 on CDs at R1
- Not all quests are made equal
- Level brackets (5, 7, 12, 15, 20+ etc)
Wiki might have "some" data, but its...