I was mentioning this to Hyrion last night and will pass it along here as well for those interested.
One of the add-ons that I use for tanking is Scrolling Combat Text (SCT). I turn off all my outbound damage and leave on only my inbound damage.
The nice thing about SCT is it will show the damage of each incoming hit, which ones are crushing blows (marked with a ^), and identify spell damage by putting the spell's icon beside its damage number. For easy reading I set the display to 'Rainbow' (number arc out and down, alternating left and right).
On rage-starved fights, assuming the healing situation is tacked down, allowing a crushing blow through is helpful to maintain TPS. Once a ^ marked damage is received, move back to activating Shield Block.
SCT is also helpful in breaking down what damage is physical and which is spell based. A helpful spot is getting positioning on the inner edge of the ring on Lurker to pickout when you start taking damage from the boiling water, as well as watching out for useful applications of Spell Reflection.
Check it out, see if you like it. You can always disable or uninstall it. SCT is very useful for understanding boss damage output better and making better quick decisions on increasing or decreasing the mitigation of incoming damage.