From Diablo Wiki
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This formula basically means that if you have less than half of your stat points into [[Dexterity]], you will have a lower block rating than your shield displays, but if more than half of your stat points are in Dex, then you will have a higher blocking %. Blocking is capped at 5% minimum and 75% maximum and displays if you hover on your Defense in the Character window. Sorcs and Necros will almost always be blocking at 5% past Clvl 50, so should pick shields based on their magical bonuses, not for the blocking percent.
* Blocking is reduced to 33% of this value when
attacking or running, but uses regular values when walking or standing still.