Basically once you get to 102.4% you want to start replacing block rating with stamina, dodge, and/or parry WHILE MAINTAINING 102.4%. This is when you can start getting a little creative with your gear.
Once you get past 102.4 your extra block rating "falls off the table" so to speak so those stats are considered "wasted". This is somewhat misleading because they only fall off the table when you have holy shield up and/or redoubt which isn't ALL of the time (though it should be if your rotation is correct) so there is a little bit of grey area.
Also at this point you can start making other gear sets that emphasize spell damage and block value to use when you overgear content or are tanking things other than raid bosses that don't require you to have 102.4%. 102.4% is only required for level 73 raid bosses that crush. Some of them (such as bear and lynx in ZA) you don't even need 102.4% because those bosses just don't crush (because their special moves basically hit as hard as a crush anyhow) and you can dip below that in favor for more stamina and avoidance.
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