Mmm... if that's for calculating when Sbar > Sblock or viceversa, avoidance and mastery gearing have a very heavy value on it. Did a miss it on the cells? Also, whether the LGM has procced, and of course Demo Shout and Shield Wall have a heavy saying on which one will prevent more damage.
In the end, most of the times you grab two auras, one that tells you the total external absorbtion you have up, another that tells you how much of your Sbar have left. Then you ponder: do I have a good external+internal absorb? If yes, Sblock or sit on rage, if not: I'm going to die or drop significantly? If yes, Sbar it, if not, Sblock. You may end up entering a loop where you can't build enough rage for a Sblock and need to 20 rage SBar to survive, but it's your healers and CDs role to get you out of it.
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2) Devastate and Shield Slam crits gives you rage. Rage is mitigation, that's the role of crit in mitigation. If you tank normal, play with whatever you find fun. If you are used to mastery stack, try avoidance and see if you like it. Personally, I wouldn't tell anyone to pick avoidance after going heavy with mastery. They don't mix well, go crit after mastery (or before mastery) if you decide to gear that way.