The GGF "guidance" is viewing from a certain distance in certain conditions - this is mainly for scratches and is the industry's way for suppliers and installers to wriggle out of changing the units.
If it is noticeable, it should be changed and its not up for debate!
You wouldn't accept a scratch on a new car that you couldn't see from 7 feet!
Same goes for the ceiling. If you need a level to prove it is out, it is probably not too bad!
As we say in the industry, if it looks right, it is right.