Chance to Block = min( (Shield+Bonus) * (Dexterity-15) / (clvl*2) , 75) %
Shield is displayed ctb% on the shield
Bonus is added ctb% from Guardian Angel, Holy Shield, Safety amulet, etc...
clvl is your character level
You can work the formula backwards to figure out the needed Dexterity to reach 75% block