Inspired by this, this, and this, I just processed this list of monsters to get the average (and standard deviation) AC in monsters, by CR. It comprises all the SRD monsters, I believe.
A common DPR chart shows the average DPR of a character at a given level. DPR often depends on the to-hit bonus of characters and the AC of monsters. For example, at level 10, Bob can either:
- increase his damage bonus by +2
- increase his to-hit bonus by +2
We can't justify which one is better without knowing how often Bob hits his attacks against his enemies.
Can this graph be used in DPR charts? Ideally, it could be translated to find, for a character of a given level, the average AC of the monsters he faces.
One way would be to consider the character's level to match the CR of enemies. So, for example, Bob would calculate its accuracy against an enemy of CR10. Bob has a +5 to hit, does 9 damage per attack, 3 attacks per round, and a CR10 monster has an average of 18 AC.
- With +2 damage, Bob has 40% hit chance, 11 damage per attack, and 13.2 DPR.
- With +2 to-hit, Bob has 50% hit chance, 9 damage per attack, and 13.5 DPR.
If a MonsterCR to CharacterLevel matching isn't adequate, what would be? Can we use standard encounter-creation rules to build a chart that matches player level to average enemy AC?