I believe it looks something like (Base AR + Straight AR) * (% AR), but I could be wrong there. In any case, base AR is:
(Your attack rating without +AR or +Ar% from equipment, charms or skills)
= (Dex * 5) - 35 + ClassBonus
The ClassBonus is:
20 for Barbarians and Paladins
15 for Assassins
5 for Amazons and Druids
-10 for Necromancers
-15 for Sorceresses