I'm actually going to go so far as to suggest that it NEVER makes sense to use ex-heal. If we didn't have reasurecrion, you could say that using it with low health would make sense.
But, since that bar is going to be converted into extra health automatically (unless we choose to opt out), i don't see why you would ever choose to spend a bar for 9%, when you could spend that bar on getting a 10 second, 1.5x damage multiplier that breaks armor (even x-rays!). To me, ex-heal would have to give us something like 15% back for it to be worth it.