I dinged 73 last night, and scribed this spell. Our tank had 13.8k hp at the time, so we did some tests. I cast this spell when he got to 50% of health, and at one point it went off and healed him for 8500. Only once. The rest of the night, it would heal him for 11k, 12k, 10k, well over the 8500 point range.
During this time, when I cast it, I could then refresh tenacity on people, or the spell haste, or cast a nuke here or there, etc. I think it's going to replace the complete heal in my spell line up hehe.
Not sure where Benjiyardo is saying it heals 45%, unless Sony changed it from when he posted. It certainly healed my tank fully all times but once last night (10/28/06).