As I've been playing Hack'd & Slash'D more, I'm finding that the armor class ratings tend to be too low. A suit of leather armor only allows you to reduce damage 5% of the time... and a suit of plate mail is only effective 25% of the time. At the very least, I want to double these numbers... this would make leather a default rating of 2, and plate mail a default 10 (50% chance of success).
This would give plate +3 a rating of 13, making it 65% likely to cut combat damage in half. That's better...
I could swap things out a bit and get rid of the layer of armor between chainmail and plate, and move the basic tiers to every 3 ratings... this would mean:
Leather is AC 3 (15%); Studded is AC 6 (30%); Chainmail is AC 9 (45%); Plate Mail is AC 12 (60%). This means there are two types of light armor and two types of heavy armor... it also means that a suit of plate mail +3 would have AC 15, giving the wearer a 75% chance of succeeding with an AC check.
Furthermore, I have not been applying the level difference modifier to AC checks (upon review, the rules aren't clear about whether you should or not, but I always assumed you didn't - you are resisting unleveled damage, but a level-based attack), but maybe I should (you would be more likely to be able to use your armor effectively against a goblin than you would against a dragon, right?). I like how this alters combat based on level differences - your armor is pretty awesome, until you fight a foe 4 levels higher than you are, because you are 20% less likely to absorb that damage. At level 6, your suit of chainmail +2 is very effective when you are being fired upon by hobgoblin archers (it cuts damage in half on a roll of 16 or less, so 80% of the time).
This means I have to modify AC values for monsters as well to scale them to the new ratings.
The other benefit is that I can create all kinds of layers between the existing armors... a local smith could forge suits of armor from the giant ants that populate the area. This is heavy armor that is slightly more effective than chainmail, so it has a default AC of 10. It's not magical, so a suit of ant chitin armor +2 has AC 12. This ends up giving a lot of gradation between armors.
One more takeaway: this means that a level 10 wizard is going to have AC 10 against goblins even with his simple traveling robes... because he's a level 10 wizard. No matter what, you don't reduce armor below 0... and you always take half damage on a natural 1 AC check (so even unarmored characters get to check).