Block-capped, unhittable, uncrushable, combat table coverage -- all these terms (some deprecated, some still relevant) refer to the same basic principle: achieving 102.4% block and avoidance in order to push normal hits off the combat table.
Now, what I just said in that last sentence might be an arcane mishmash for some folks who are new to tanking, and I aim to fix that. One of the most potent things you can do for your gearing is hit that magic 102.4% number, to achieve full combat table coverage. Doing so, you're looking at least a constant 40% damage reduction for any melee hit. This is the holy grail for shield tanks right now. Let's talk about how to make that happen.
First, a history lesson
The first I ever heard of the combat table was years ago during The Burning Crusade. Back then, paladins could ostensibly raid tank, but first they had to accumulate an obscene amount of avoidance and block rating. Thus, if they slapped the old Holy Shield (+30% block chance buff for 8 hits) on top of their miss chance (5% plus whatever they got from defense), they'd push crushing blows off the table.
You see, back in the day, raid bosses would attempt crushing blows, which were 150% damage attacks. Only by filling up the combat table and pushing crushes off it would a paladin be able to effectively tank a raid boss without being quickly ground into a fine, sparkly pulp. Of course this was all dependent on Holy Shield charges not being used up early and not getting feared, or silenced, or all other sorts of fun things that made paladin tanking back in BC an absolute, unmitigated joy.
I hope the italics there properly convey my sincerity.
Then in Wrath, crushing blows went away, but CTC assumed a new form: block capping. This especially came to prominence in tanking the adds from the Anub'arak encounter in Trial of the Grand Crusader. A shield tank with full CTC could trivialize the adds and was pretty much required for a heroic kill when the fight was current content.
After ToGC, Blizzard stopped itemizing gear with block chance, so the only way to cap was via lower-ilevel pieces. People seldom attempted to block cap in Icecrown, due mostly to gear restraints, so the whole practice fell out of style. Of course, mastery and Cataclysm have changed all that.
But I digress. Now that you've all indulged me, I should probably explain how this all matters to you.
What the heck is the combat table?
The combat table is a range of possibilities of what the result of an attack against one entity by another can be. In our case, we're specifically interested in how an attack against us by a raid boss will pan out. The combat table for an attack against a player by a raid boss always includes these five items in the following order of precedence: miss, dodge, parry, block, critical hit, crushing blow, and normal hit.
The player's miss chance is always 5%. Dodge, parry, and block come from the character sheet as a result of your stats. Critical hit is automatically pushed over as a result of the Sanctuary talent. Crushing blows only can be performed by a mob four levels higher than the player; with regards to a level 85 player, those mobs don't exist, so you won't encounter these unless you jury-rig the Caverns of Time to send you back to a pre-3.0 ruleset. And finally, normal hits are the remainder, a fungible number made up of the table leavings from all the values that have a higher precedence.
What this means is that as the higher-precedence values increase, the lower-precedence ones can be completely "pushed off" the combat table. As your dodge, parry, and block increase, your normal hit chance concurrently decreases. Eventually, once all your avoidance and mitigation stats meet and surpass the 102.4% mark (this number is more than 100% because the raid boss is technically three levels higher than the player), normal hits are impossible.
So, for example, let's look at the combat tables of three hypothetical tanks. The first one is Dan. He's just starting out -- or perhaps he's a kind soul who off-spec tanks when the normal tank is off sipping peppermint martinis in some Paris bistro -- has the normal 5% miss (the "m"), some dodge, some parry, not much mastery. As a result, he's not covering much of the combat table. Dan is going to be taking a lot of normal hits.
The downside: The mastery cap
While it's great that we can push normal hits off the combat table, keep in mind that means that we can also eventually push blocked hits off the combat table -- though not completely.
This, unfortunately, means that mastery has a cap. The cap is mutable and depends on how much avoidance you have, but basically it works out to 97.4-(dodge+parry). So if you have 30% combined dodge and parry, your mastery cap is the amount of rating it would take to achieve 67.4%. Anything over that is wasted itemization; it falls right off the combat table. And any additional dodge or parry you add makes that cap shrink.
At that point, you'll want to start shedding mastery via enchants, gems, using different trinkets, whatever. While this may be a problem for top-tier raiders at the moment in T12 and T13, it's going to become more and more common for prot paladins to approach mastery the same way a DPSer might approach hit. You reach that magic number, and then every point afterwards is looked down upon like some horrid curse, thanks to its inherent uselessness.
While the concept of CTC and the corresponding 40% damage reduction is truly drool-worthy, there are some strings attached. For one, to be obvious, you can only block melee attacks. (I know, I'm as shocked as you are.) So you can't depend on mastery to cover your rear end when Al'Akir is tossing lightning bolts at you or Nezir is unleashing a frost breath or Cho'gall's melee attacks are being burnished by a fire elemental's power. Basically, any magic-heavy encounters are going to dilute the efficiency of total combat table coverage.
Moreover, for any fight in which you can't guarantee that your front will constantly be facing what you're tanking (I'm thinking an add tank on Cho'gall specifically), you won't be getting much benefit from CTC. If your back is facing a mob, you're not going to be blocking anything.
As good as mastery is, there are always going to be outliers for which it makes more sense to have a higher health pool at the expense of a higher block percentage.
A tool to maximize your CTC
I wrote an Excel spreadsheet for calculating the value of an item in terms of CTC. I have it available for download here on my blog. (Moreover, I invite you to amuse yourself at checking out my hideous, roundabout way of calculating all this stuff.)
The function of the sheet is to compare two tanking items -- like, say, Ironstar Amulet (1.18% CTC) versus the Necklace of Strife (2.32% CTC) -- to determine which one makes more sense for you. I'll spare you detail instructions on how to use it in this space, but if you're curious, you can check out my spiel on the blog post accompanying the spreadsheet.
Wait, did you say agility?
Sure did! Agility necks, rings, and cloaks can (depending on the piece) be excellent for your CTC set. By agility pieces, I mean something with mastery and then a threat stat (like haste or crit). When you run a piece through the spreadsheet -- like the Necklace of Strife example just above -- you'll find that agility plus mastery can add up to a potent combination. Even when saddled by a useless threat stat, these can work out to more combat table coverage than something that is technically considered a tank piece.
Each point of agility is worth .00328% dodge before diminishing returns. While that seems minuscule, it adds up -- especially when a piece can have 190 agility, which alone is worth .62% dodge before diminishing returns. Definitely nothing to sneeze at.
In any case, don't overlook agility pieces just because they have agility. Often, you'll find they're a much better fit for you than something ostensibly itemized for your role.
The future of CTC
There's still a lot of Cataclysm left, and I'm not entirely confident the situation will remain as is for the remainder. If complete CTC is possible in high-end raid gear this tier, then thanks to ilevel inflation, it'll be accessible to a majority of tanks in normal raid gear next tier.
How will Blizzard respond to this? Will it nerf our mastery yet again? Will it follow through on the threat of giving later tier bosses expertise? Will it introduce a Flamewell Radiance? Time will tell, I suppose.
The Light and How to Swing It tries to help paladins cope with the dark times brought by Cataclysm. Check out our protection 101 guide and our suggestions for protection paladin addons.