I think this is being misinterpreted a little bit. This only blocks attacks that exactly equal the final number. So for their example, AC 14, and final total being 23, 15-22 still hits and 24+ still hits.
It’s an interesting magic item but ultimately it’s too much additional math and dice for a turn.
Looks like the roll is done at long rest, so it would be a relatively static number.
For a real short example, you roll a 10 after math, so you just put a little 10+ac misses automatically note somewhere handy until you sleep and roll again.
It’s a fun little ring, and could be a really hype moment if/when it hits
The crit line seems a bit hasty. Enhanced crit ranges exist on several monsters, and that makes this magic item even less worth using. Preparing for a particularly powerful foe by bringing your uncanny AC to what you expect to be a 19 on the die is the best use I can find. However, “this does not negate natural 20s” is very fair.
