It really all depends on what level Holy Shield and what level you're going to be.

Assuming level 85 and +35% block from Holy Shield, you'll want about 114 DEX.

Here is my math, so you can check for errors:

Dex=(Clvl*2)*T+15
____________
B

Dex = 114
Clvl = 85
T = Total blocking (40% before Holy Shield + 35% from Holy Shield = 75%)
B = 82% Blocking on HoZ (Base 52% on Zakarum shield + 30% Chance to block from HoZ mods).