New Prot Warrior Tank and _when_ to focus on Mastery Reforging
It sounds like you have some confusion about tank stats.
The number that you referred to (102.4) used to commonly be called "unhittable". This means that you will never take a FULL hit. The term I see and like for it now is "combat table coverage", sometimes just said CTC.
When a mob swings at you, it is basically like rolling a 100 sided die. You are trying to ensure that they have no chance to "roll" a hit.
5% base chance for them to miss
+ dodge% on your character sheet
+parry% on your character sheet
If you can get these to add up to enough to cover all 100 sides of the die, you will always mitigate at least some of the damage.
Due to the more complex details of how it works (due to the boss being 3 levels higher than you) you actually have to cover 102.4%.
Dodge and parry are subject to diminishing returns. This means the more rating you already have, the less % an additional point of rating gives you. Mastery does not suffer from dimishimg returns and increases the chance you block.
Because every rating point of mastery has the same value no matter how much you already have, it is the stat that is easiest to stack up to get to the combined 102.4. Also, blocking reduces damage by a base 30% which allows healers to use more efficient heals.
Mastery is always a good stat because it smoothes damage and is never reduced by dimishing returns.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng