I recently acquired an item that mimics the spell Deflection(adv players). How high does my AC need to be in order to reliably(80+%) deflect the attacks of CR16-20 monsters? Assume my character is built to take aggro for the party
I believe the rule of thumb with ac is 10+level to not be autohit by mooks, 15+level for most characters, 20+level for tanks, 25+level is slightly excessive, and 30+level for pissing off the GM, but I could be remembering it wrong.
Looks right to me.
The link you posted does not go to any file, you might want to check that url.
I did a post about this in another thread here.
Plugging in your numbers (cr 16+ and 80% miss) you're going to want an AC of 27+(target cr). Meaning AC 43 at lv16 and increase it by one every level.
Alternate math:
AC 43 = 80% miss chance vs cr16
every point of AC over this = +5% miss chance (max 95%)
every cr over 16 = -5% miss chance (up to cr23)
That kind of AC is going to require maxing out all the common options, a shield, and maybe a few more shenanigans, though.
This isn't mine.. there is a wonderful guide for benchmarking and such around the forums if you look for it.
https://docs.google.com/spreadsheets/d/1CCxnAb8apicr3fOrSCEFNRwHlzRieMrXm6l d9-uLAFc/edit#gid=0
My general rule of thumb is to be a good tank you want 15+1.5*level AC.