It's a great suggestion and I hope they finally do something about the range of useful ac.
As The_Brave2 mentioned, a uniform distribution does have some shortcomings tho (notably also at lower lvls). This could be circumvented by doing something like
1. d3
If the result of 1. is
1 - mob rolls to hit + d40
2 - mob rolls to hit + 20 + d20
3 - mob rolls to hit + 20 + d40
This would make the middle range more probable. Just a thought. This too would result in problems at lower lvls. It could be possible to replace the 20 in case of initial d3 with cr tho. Guess that would be better.
Heh, getting slightly amused so I'll type another possibility involving cr
1. d3
If the result of the d3 in 1. is
1 - mob rolls to hit + d40
2 - mob rolls to hit + roundup(20*(cr/maxcr)) + d20
3 - mob rolls to hit + roundup(20*(cr/maxcr)) + d40
So for maxcr 30( not sure about current max mob cr in game. Just an example), cr 30, to hit 60, the range becomes 61-120 with values more concentrated in the middle make it a bit more ... how do I say it... less random? Edit. more expectable? Sry my english Second edit. Removed an inconsistency in the second model. Too bad it got completely ignored again
Third edit^^ After playing with a spread sheet, I repaired an error. Now assuming max cr, and mob to hit 60 results in ac 61-80 each point giving 0.833%, ac 81-100 each point giving 3.333% and ac 101-120 again each point giving 0.833% damage mitigation. My suggestion for critical would be the same as it's now, the highest 5%. Meaning if the range is 60 points, critical occurs at 58, 59 and 60. If it's 40, then critical occurs at 39 and 40. As for glancing hits, again the upper third seems ok. Similar to what is now. Guess I'll make a new possibly futile post about this to make it at least slightly possible that this is noted.