Your first iteration is never going to be perfect. You'll have to do a lot of play testing to see if the damage and armor formula you're using matches the vision for how you want the game experience to feel. So... just implement it and try it out! Does it work? Is it fun? What kinds of interesting doors does it open up in design? Don't overthink this stuff too early. Get something working so that you can iterate on variations of the idea until you land on something you really like.

for things like this, the goal is to model the desired behavior as simply as possible.

if what you have yields the desired behavior, and no simpler model / formula / set of rules does, then this is the best choice.

if it does not yield the desired behavior - its useless.

if a simpler model will do, its unnecessary complexity / over-engineering.

so you have to start with the definition of the desired behavior.

One thing that's always bothered me about games with medieval warfare and armor and damage calculations is that they rarely ever actually realistically model the effect and purpose of armor.

I remember going to a museum in Finland a few years back and looking at weaponry and armor from 500+ years ago. Knights were covered, head to toe, in steel. Almost nothing could penetrate that. Then they also have a kite shield and sword. A large squad of these guys, slowly marching towards you, was an invincible, inevitable death. The only way you could kill one of these soldiers was to tire them out, lift up their arm, and stab them with a very thin dirk beneath the armor. People didn't wear armor to take LESS damage, they wore it to take NO damage. Even a good scratch or cut could get infected and take months to heal, so people probably spent a lot of effort trying to develop armor which made them impervious to damage and used weaponry specialized against armored opponents. A mere peasant armed with a pitchfork would be minced meat, no matter their martial skill. Armor was expensive though, so not everyone on the battlefield had it. To wear armor was almost a status symbol, and to make it even more of a status symbol, some rich nobles would have their armor inscribed with decorations. One other surprise is in regards to leather armor. Leather armor is HARD. It's not that soft, supple leather which is designed for clothes and couches, it's leather that has been treated differently so that it hardens into a protective shell. It's not as hard as steel, but it will do a very good job of blocking most incoming attacks.

If I ever go and design a game with medieval combat mechanics, I would certainly try to take into account the realism of armor and its functional use. There would be no "Hey, I'm fully armored and yet a single sword swipe cuts me down!" bullshit. What's the point of wearing armor then? Might as walk around the battlefield naked because it's less encumbering and hot. Let's talk about heat and weight a bit too. I don't know about you, but I've had to wear full modern armor before (flak jacket, SAPI plates, kevlar, side SAPI, crotch cover) in the desert. It's bulky. It's uncomfortable. It's HOT. It's like wearing a snow suit in the middle of july. You don't want to move around much because its heavy and tiring. So, designed game mechanics could take advantage of this fact: The temperature, weight and amount of armor contributes to exhaustion, and exhaustion is a function of fighting capability (blocking and delivering blows). A part of battlefield strategy then becomes managing exhaustion, which is what was actually done! Lines would frequently rotate from the front to the back, rest and recover for a bit, move towards the front, and then resume fighting. Gradually, everyone gets really tired and more R&R doesn't help. But then the army which wins is the one which can bring in fresh reserve troops, preferably in a flanking maneuver. The enemy is too tired to fight fresh troops, and they're being flanked, so they either change focus to fight the fresh troops or continue engaging with the tired troops while the flankers mop them up. It's a fascinating mechanic :D

So... damage would be a function of the martial skill, exhaustion, equipped weapon type, sustained injuries, strength. The frequency of attacks would be a function of exhaustion, martial skill, and morale. An incoming attack wouldn't just be subtracted from the armor of the victim. That would be WAY last. First, you'd want to give the defender a chance to dodge, block or parry the attack. This would be a function of their martial skill, morale, exhaustion, equipped weapon type, mental focus, incoming weapon type, and sustained injuries. IF an incoming blow can't be dodged, blocked, or parried, then it is a hit. The hit would be registered somewhere on the body, but the actual damage inflicted would be a function of how armored that body part is and what type of damage we're inflicting (blunt, piercing, slashing, magical, fire, etc). The design goal would to make it so that an armored opponent *could* be near invincible. That's scary, and it should be scary. Nobody fucks with armored guards (see: swiss guard). If you're not prepared to fight an armored opponent, then you lose. Likewise, if nobody is prepared to fight your armored characters, they lose. Game "balance" would be less about balance on the battlefield, and more about access to resources prior to battle, so battlefield victory is partially determined by preparations before battle begins. (man, I want to make this game...)