Physical Damage Resist

Armor Rating works on a one for one scale. 1 armor rating = 1% damage reduction.

The skill "Pain Tolerance" does not reduce your damage taken. I'm guessing it reduces the amount of max hp you lose from damage, along with the stun resistance it gives you.

There are mods that add armor rating or durability, they are flat values and straight forwards. You can add up to 15% mobility and 15% damage reduction with the highest tier of each in your armors.

The difference between light armor appears to be that heavy armor has 10% more damage reduction but gives up 10% mobility in tradeoff for each tier. Heavy armor does get more effect resistance per tier though.

Once you are in T2 or higher armor and start to put skill points into light/heavy armor as well as natural healing you essentially become unkillable. Your survivability might scale a little bit too much. Zombies were doing like 2-5 damage per hit when in T3 armor and skilled up. You essentially stop needing to even heal yourself. This may be appropriate for higher difficulties but on nomad you're a walking unkillable tank.