Then the ONLY way that can be done successfully and consistently is with a level. The level would be CRITICAL to such a setup. So your assertion the level doesn't matter winds up going right out the window.
Arthur....thats just BS. My assertion (by your own admission) is correct. By your way of thinking.....the bubble would have to be in the EXACT same location between the bubble window for the level to work, then. ANY slight variation would throw off the shot (however slightly or pronounced).
What did people do before levels on sights? They tried to maintain a consistent form when sighting in and shooting. Right back in the window......