well, law 15 tells us:
[LAWS]Ending a ruck
The ruck ends and play continues when the ball leaves the ruck or when the ball in the ruck is on or over the goal line. [/LAWS]
So to me that means all of the ball must be outside all of the body parts in the ruck. Nothing about ball off ground, etc
But that doesn't help us with the scrum, does it?
In reality, I manage the scrums as per the ruck requirement (ie ball completely clear of and beyond #8's feet). I think it is unsafe for 70kg scrum half to be poleaxed by a 90kg flanker while the scrum half is in an unprotected & vulnerable position trying to dig the ball out. If I think the scrum half is taking too long, I'll manage that.