Dont know how your dnd experiences go but players can easily know what the AC to beat it. Some DM's just outright say it so the players know upfront it they hit or not to make combat go faster. In other cases the players find out themselves.
If a 13 misses and a 14 does hit its pretty easy to know what the AC is...
I personally think a percentage is fine but a 13/20 needed to hit could also work I suppose? So long as its clear to the player which target is easier to take down.
