You bring a very interesting question in how DP factors in block% to their EHP calc. The SS adds 1137 armor and 75 AR to your total plus any armor from any net str gain over the EF. It also has 5% DR to melee which DP counts 1/2 toward your ehp.
Given all that, it looks like your EHP should increase about 32% not counting block, but it's actually increasing about 89%. If block was fully effective, your ehp should increase 1.32/.66 -1 = 100% increase. So block isn't fully effective but is mostly effective in the formula. This is a rough estimate since I don't know the stats of the EF, but should be in the ballpark. I also don't know if red dmg vs elites is included in ehp.
What DP probably does is assume a certain level average incoming damage and bases block's effectiveness on what leak's through if the block is effective.
I'd be interested in testing this a little further. Baller, Nikeboy, powpowboom, Crypter, and any other godly S&B, can you at some point post your EHP with the stormshield equipped and with a 2nd weapon equipped? I want to see if I can derive what incoming damage DP uses for block purposes. If they haven't changed their formula in a while, it may be outdated with the new MP changes. Block is more effective with low level attacks and less effective with high level attacks.