First off Block is not avoidance. don't call it avoidance, it's a theorycrafting thing, calling block avoidance is just bad. like turning you back on the boss bad. We call Block+avoidance chances Combat Table Coverage which is usually abbreviated to CTC. so tyou have 100.23% CTC, not 100.23% Avoidance
That hurdle out the waylets compare metagems, ignoring the 81stam the Eteranl increase you block values for 30% to 31 mitigation and your critical blocks from 60 to 62% block values. essentialy its increase the amount of damage you block by 3.33% .So assuming a boss hits for 100 damage every seconds and you have 0% avoidance and 60% block chance, and 40% critical block chance. Becuase critical block doubles our block values you can essentially treat it as a +Block value, so 40% crit block means on avareage we block for 42% of the damage on average (30% x 1.4). so we have a 60% chance to reduce the damage by 42% so an overall damage reduction of 25.2% so the incoming 100 DPS is reduced to 74.8 dps
if we plug our Eternal gem in we have to increase the block values by 3.33% so we'd now have a 60% chance to reduce the damage by 43.4% (42 x 1.0333) for an aoverall damage redcution of 26% so the incoming damage is reduced to 74dps.
If instead we plug the Fleet metagem in we get an extra 69 mastery which increase our chance to block and critically block by 0.58%. so 60.58% chance to block and a 40.58% chance to criticall block, on avearage we block for 42.174% of the damage and doing that 60.58% of the time means we avarage a 25.5% damage reduction. so our 100 incoming DPS is reduced to 74.5iDPS
So the Fleet meta gem works out to be 0.6% more damage taken than the Eternal, avoidance (parry+dodge+miss) will decrease this difference and the Fleet will finally be equal to the Eternal at the point you reach 100% avoidance.(parry+dodge+miss).
PLay around with the calculator i posted its pretty instructive that once you hit the point where you cap reach 102.4% CTC with shield block Up the Austere falls behind the Eternal, unless the largest damage source is unblockable physical damage. And if the majority of damage is spell damage since neither the Eternal, Austere, or Fleet have any affect so the Efflugent wins.
If the Majority of the damage is Blockable use the eternal
If the majority of the damage is not not blockable but physical use the austere
If the the majority of damage is spell damage use the efflugent
Every fight in Dragons soul except Hagara HC, Yorsahj and madness the majority of the damge is blockable. and even Hagara falls into that too if you don't eat her focused assaults.