Diablo III: Reaper of Souls

However, the closer you are to 100% reduction, the more EHP you gain for that +0,5% so it balances out. There's no diminishing returns.

The formula for the reduction is:
EHP multiplier = 1 - (1 - AR/(AR + 350))

So taking a base HP of 1000 I calculated the effective HP for increasing amounts of AR, here: