|At least this is what TC Electronic says.|
"Studies have shown that 95% of the viewers will change the volume on their TV or receiver if the loudness jump or difference is an increase of 5 LU or decrease of 8 LU."
That seems like a pretty important thing to keep in mind I haven't heard anyone else mention.
For the real details check out this paper.
I really need to get all of this information into my head, and figure out how to actually do it.