Shield Blocking

Blocking with shield isn't as straight forward as looking at the value stated when you hover your cursor over the shield.
Bolty published his findings in Diablo II: LoD Beta Report #1
Instead, the chance to block is given by

(Block * (Dex - 15)) / (Level * 2)

Block indicates blocking chance shown item,
Dex indicates player's dexterity,
Level indicates player's level

There is a minimum chance of 5% and a maximum chance of 75%.
When running, that chance is further reduced to a third of the normal.