In what sequence is damage reduced (armor, absorbing shields, %reduction, amumu's E passive component/Sion's passive/Defensive mastery/Harden Skin)?

The explanations of the masteries Strenght of Spirit and Expanded mind are a bit confusing.
"Increases your champion's health regen/mana by X% of your max mana." Could someone explain what your mana refers to?

Don't know the formulae of damage reduction. The mastery, however, I think just means it takes % of your champion's max mana and turns that number into a HP per 5 second. Don't know what else it could possibly mean.

For Strength of Spirit, it's health regen per second. Example, if your champion has 400 mana, it's 0.3% of that in health regen per second, so 400 * 0.003 = 1.2 health regen per second, or 6 hp5. It's a very powerful mastery for earlygame laning, but becomes progressively weaker as the game goes on.