What is the proper loudness for listening?

It’s really up to you. I listen at a variety of different levels based on what I want to hear, how I want to hear it, if it’s emotional to me, if there are other people listening with me, etc etc. Also listen so that you can hear the music clearly, define the soundstage, and enjoy the material. 

Though this won't include Rock or some Acid Jazz, it is simple for me. The perfect level is found when you look for the soft passages, often found quite noticeable in classical music, and set that at a comfortable but yet soft volume i those portions of the piece, the rest takes care of itself. This after a while and a little practice, this  will come naturally. For those that think that music is good if it is loud, I can only offer my pity, Volume has noting to do with sound quality, with one exception. My speakers and I assume most quality speakers don't even 'Turn On' till they get a certain power level. This means that Yes they will play sounds at lower power levels but their excellence doesn't shine until that minimum threshold is reached. This is going to be different for every speaker made and is like in my example above, you set your volume higher to achieve this quality threshold, you may be pushing the limits of crazy when the more powerful portions of a piece come along.
What does all of that mean? It's totally up to your own particular taste and what wifie will permit before she gets the cast iron skillet out to clobber you with.

For me 70 - 85 dba and I am more than happy.

I am surprised how loud some are able to listen.

I use a dB meter, C-weighted, fast response.

75 dB peaks when my wife is sleeping in over the weekends.

85 dB peaks after she has joined the living, and it is LOUD to me.

I have learned that the best way for any system is to turn it up till it starts to get harsh, then turn the volume down till everything comes into FOCUS like a lens.You can tell pretty quickly. I heard a LOT of systems at AXPONA 2023 with my wife and we both thought most systems we heard were too loud for the room. But, there are people coming in and out, talking, etc. So it is limited to say the least.