Scars Unseen
Hero
Hmm.. To keep things from slowing down at the table, rather than make it an automatic feature and one that applies all the time, why not make it a feature of a defense oriented feat(or class feature, depending on where A5E takes things), and make it more powerful, but applied situationally. Like, and I'm throwing this out there with no mind about balancing, make a feat or feature for heavy armor types that let's a player decide once per combat, after both the attack and damage rolls are made, to ignore that damage. Call it "glancing blow," or "armor mastery," or whatever. You could come up with something similar for lighter armors. If we're making it a class feature, you could come up with several somethings, making it possible to specialize in defensive play.
Just seems to me that having something the player can use to their advantage in specific points in the battle would be more narratively satisfying than a passive addition to the combat formula. Like, it would make for a much better story to talk about the time you parried the demon general's flaming sword with your armored fist before landing the decisive blow than to mention how your armor allowed you to take 10% less damage throughout the fight, letting you stand for a few rounds more until the fight was done.
Just seems to me that having something the player can use to their advantage in specific points in the battle would be more narratively satisfying than a passive addition to the combat formula. Like, it would make for a much better story to talk about the time you parried the demon general's flaming sword with your armored fist before landing the decisive blow than to mention how your armor allowed you to take 10% less damage throughout the fight, letting you stand for a few rounds more until the fight was done.