There's also heals which need to be factored in.
I know we're talking about a worst-case scenario here, where there aren't heals incoming, but realistically, especially in a 25-man, it's horribly unlikely that you won't have hots rolling on you at the very least, even in a worst-case scenario. Your EH set may not have enough health to take 3 hits without heals...but what if some hots tick, and/or you get a last-second, somewhat gimpy heal that tops you off enough to survive a third?
That possibility is more likely to occur when you are focusing more on EH than avoidance.

