I just have two questions for you coming from someone who also plays WHM heals....

1. Why are you waiting to the point that a cast bar would be too long to throw a heal and also too long for the animation of Benediction to happen before you actually pop it? I agree, it says Instant and I get your frustration with it, but if you are waiting that long to pop it when you really should have popped it a GCD ago, the Tank was dying anyway. Also, I've had a Lustrate on my SCH hit 0 because I waited too long and the Tank died as I hit it, also Instant, but because I didn't allow for the animation, I was late.