I realize I'm presenting this without any proof but I think you'll find it works.

Avoidance = (T from table) + (Defense * 16/9), but never less than 0
Caster Mitigation = (Buffs/3) + (Defense/2) + (Equipment + 1)
Melee Mitigation = (Buffs/4) + (Defense/3) + (Equipment * 4/3)
Displayed AC = (Avoidance + Mitigation) * 1000 / 847

Caster means Enchanter, Magician, Necromancer, Wizard.
Melee means all classes except Monks and casters.
Defense is the character's Defense skill.
Equipment is just the sum total of the raw AC from everything worn. Don't forget the range slot.
Buffs is the sum of the bonuses from all buffs. You'd have to check your spdat file to see those numbers, but for reference Skin Like Wood adds 15 and Skin Like Nature adds 55.
T comes from the table below, use the column corresponding to the character's level.

AC is calculated with all whole numbers, so each and every time there's a divide the result is truncated. Truncated means any fraction is dropped as if it didn't exist, not rounded up to the next whole number.

I'm not entirely sure that all the factors I put under Mitigation belong there, but this is how it looks. It's even possible that Avoidance and Mitigation aren't truly seperate. Someone could have decided near the end of beta to start using (Avoidance+Mitigation)/2 in all avoidance calculations and someone else could have decided to start using (Mitigation+Avoidance)/2 in all mitigation calculations.

There are several situations where this formula won't work. It won't work quite right for monks or iksars but the agility table applies to everyone, even them. The iksar bonus seems to be based only on level and added to mitigation, but I don't know the exact amount. It won't work quite right if the character's drunk. It also might not work quite right for naked characters who've been debuffed, due to a safeguard similar to the one that keeps avoidance from being negative. But in general this works for everyone regardless of hunger, encumbrance, injury, skills, or any other factors.