What is the proper loudness for listening?

There is no one volume.  Brian Eno made recordings where he prefers 

the volume level to drop off completely.  Then there's Kuma's Corner, a bar in the Avondale neighborhood in Chicago where the proper volume is ear bleeding, brain damaging loud.  Fantastic burgers and beer and drinks choices.  Just bring earplugs and come on a warm night when you can sit outside.  You'll still need the earplugs though.

What is the proper loudness for listening?

                  Whatever floats YOUR boat.

Your age!

60-70’s on the Decibel Meter ap set on “C” for me. I guess I have young ears…80’s + just too much of a good thing and need to left to short stints.

9 to 11 clock position.

Your gear, your speakers, your room along with treatment (or lack thereof), the equal loudness curve along with your personal preference all play a role.