The Elder Scrolls V Skyrim – Does Magic Protection Protect Against FUS RO DAH?

If Tamara Wijsman's formula is correct, hp can be factored out of the formula. Also, there's a 12/100 factor I saw from uesp.

resulting damage = damage * (1 - armorfactor)
armorfactor = 0.12 * (hidden armor + displayed armor) / 100
hidden armor = 100

Which gives these values.

50 armor -> 18% reduction -> 82% damage taken
150 armor -> 30% reduction -> 70% damage taken
317 armor -> 50% reduction -> 50% damage taken
442 armor -> 65% reduction -> 35% damage taken
567 armor -> 80% reduction -> 20% damage taken

The hidden armor in the formula is when wearing four pieces of armor (chest, head, hand, legs). If you are wearing robes or hood, or are missing feet or hands, then hidden armor is less than 100 and you get less reduction. This reduction in reduction is not reflected in the displayed armor rating.

Finally, damage reduction is capped 80%, so 567 is the maximum displayed armor rating that is useful when wearing four pieces of armor. 667 is the maximum displayed armor rating that is useful when wearing 0 pieces of armor.

If it's a multiplicative stack as others have noted, it should work like this:

Say you have a 40% shield, a 30% chest piece, and a 20% buff...

In a simple additive system, you would have 90% resistance (40% + 30% + 20%). This is a problem because a user could figure out a way to completely mitigate damage and render certain foes permanently harmless, and what fun would that be? Generally systems that use this method implement caps, either hard (user can't score higher than x%) or soft caps (after x%, all bonuses are reduced by y%)

In a multiplicative system the 1st item adds its full value (in this case 40%), but the next item only adds it's bonus onto the un-resisted portion of the damage. This works out as follows: 40% + (30% * 60%) + (20% * 42%) = 66.4% resistance. The result is a much lower effect when stacking items. This kind of system makes large-chunk items more effective than collections of small-chunk items and also guarantees that no one can ever reach 100% resistance, even without the use of maximum-caps.