It's points in that skill.
For example, to get Expert Illusion you need 75 Illusion.
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.
Best Answer
Kyne's Peace works on all animals except for Mammoths.
It has a maximum effective level of 20. Mammoths are level 38 according to the Prima Guide. The next highest level animals are Snow Bears, who are level 20, and are thus affected by the shout.
I've tested this against both, and those results seem to hold with no Illusion perks.
Haven't tested Illusion perks, but in theory they shouldn't change the result, as Hypnotic Gaze and Animage offer a combined +16 levels of effective targets, which would bring the maximum affected level to 36. Since Mammoths are level 38, they would remain unaffected.