12.31 ArP rating = 1%
hit rating highlight is base ArP without stance.
so if it says 30% you have 30% from gear/gems (this is the passive people talk about).
you add 10% on top of that to get your overall.
sunder armor reduces the base armor that you take from (think of it multiplicatively).
So 10k armor sundered = 8k armor * .6 (assuming 40% arp) = 4800 Armor left.
Boss Armor * (1-debuff effects) * (1 - Personal ArP effects including stance, mace spec, gear) = Boss Armor post Armor Reduction effects.