If you're around 910 equipped, which sounds on the low side but okayish for H ToS, you'll have around 4 million HP if you're a "normal" spec, 5 to 5.5 if you're fury / hpal.
3.5 million hits seem really high indeed, especially considering how frequent they are and how different it is from smaller raid sizes ... Can see a nerf coming to cap it at around 3M for bigger raid sizes, but it should remain higher for higher raid sizes so as to not trivialize the mechanic by simply having more healers.