Who Gets The AC?

[Rynjin]

There are two competing pressures here.

One is that AC has escalating returns -- each +1 is more valuable in terms of effective hit points than the last. Like so:

As mentioned in my post, this is not necessarily true. Even as your own graph shows, the chance to be hit can never be lower than 5% under normal circumstances (i.e. excluding re-rolls and other ways to modify hits after the attack has been rolled). That means there is a sharp point of diminishing returns; the returns are diminished into actual nothingness at the point where you are only hit by a 20.