Q u o t e:
I honestly don't think the redux is working. Spriets with imp inner fire + glyph of inner fire + shadowform should have around 52-55% phys reduction, but they're still being hit as if they're a warlock or mage. Paladins take much less damage and generally have less %. So I'm gonna assume something is wrong with the 15% from sform.
Nothing's wrong with it. Damage reductions are not additive.
Inner Fire = 1580 armor.
Imp Inner Fire = +45% armor from Inner Fire.
Glyph of Inner Fire = +50% armor from Inner Fire.
Therefore, Inner Fire is 2440*1.50*1.45 = 5307 armor.
Plus add in your base armor. 2107 is average for a priest.
Your total armor is now 7414.
7414 armor is 32.74% damage reduction from physical attacks against a level 80 attacker.
Shadowform reduces damage taken by 15%.
A melee attack that hits you would be reduced by 15% and 32.74% multiplicatively.
(0.85*0.816) = 0.57171.
1-0.57171 = 0.42829 = 42.8% damage reduction from physical attacks, or you take 57% of the total physical damage.
Compare it with mine, a paladin wearing a shield.
I have several damage reducing talents:
Imp Righteous Fury - 6%
Glyph of Divine Plea - 3%
Blessing of Sanctuary - 3%
Shield of the Templar - 3%
0.94*0.97*0.97*0
.97 = 0.8579.
1-0.8579 = 0.142 = 14.2% damage reduction.
Now we have my armor.
Imp Devo Aura = 1807 armor.
Add my armor, 26222+1807 = 64.79%
Therefore I take (1-0.6479)*(1-0.142) = 30.2% total damage from physical attacks, or my reduction is 69.8%.
So let's say a 2000 damage autoattack is coming in from an arms warrior at both of us.
You take 1143 damage from it.
I take 604 damage from it, assuming I don't block. Since my block value is 2515 with my libram buff, the attack has a chance to hit me for 0.
Notice how 604/1143 = 52.8%? I'm taking just over half the damage you are, even though my damage reduction is only 1.63 times yours. The reason for that is that each point of armor is worth the same. The more damage reduction you have, the more 1% of additional reduction is worth.
*Edited for correct inner fire numbers.