I have not heard of any exact formulas to see how much AC agi or def mods help. This may be due that we have no exact idea on what a mob hits for. Then there are mobs that hit for different amounts that would change the figures. If there is a formula, I believe different mobs have different formulas.
BTW agi does not add to AC, it is a seperate factor in the defense of a player. If you notice an item giving more ac then it says in the description, that is because if you level is higher then the items level and it gives ac there is a bonus to the AC.