Going off of Aero from my debate thread, here is what I've come up with.
Shield of fire: instant heal + armor = ((abilvlx2) + (agi/3) + magbonus)/20.
You have to admin that the calculation is pretty decent considering how it could end up. With both Shield of fire, and Fire master at level 20. With 300 agi, it comes to 107 armor bonus. With ability levels 2, agi 24, it comes out to be 10.6