Your not getting what Synapse is trying to say, check this post by Satorri out:
Basically the point is that EH does not go up in a straight line it steps up. Imagine a boss hit for 50k a swing after mitigation, there is no way you are going to survive two of those swings. AND there is no way you can stack enough EH to possibly survive two swings so EH has reached its usefulness cap after you can survive one. In that case you would be better off stacking Avoidance and praying to the RNG gods.