I would appreciate someone clarifying the percentages. For example, my Mastery rating says I block 49% (Okay), which is straightforward. However, under the "Defense" information it shows my Dodge, Parry and Block percentages. The Block % here is 54.48%, which is 5% higher than what the Mastery section entails. Therefore, my question: is the 54.48%, my Mastery 49% + the 5% chance to miss me? Or do I need to add the 5% to 54.48% + dodge + parry = 102.4%???
54.48% is just your chance to block. It does not include the 5% miss chance.
Every paladin and warrior has a base chance to block of 5%. Remember that you don't learn mastery until a certain level, so all those low level tanks would otherwise have no way of blocking!
So the 54.48% block listed is the sum of your natural block (5%) and your mastery (49.48%).
In summary, your Combat Table Coverage (aka "unhittable") calculation is
I would suggest reading this thread over at battle.net it explains how to break down avoidance and diminishing returns, probably better than anyone here at gaming.se.
From what I know, yes, the base 5% hit from bosses is covered in total avoidance (from the defense tab). That is why you aim for 102.4% cap