All throughout my childhood and early teen years I listened to music always at the quietest comfortable.
After attending my first live concert, where the pain in your eardrums is definitely a good thing, I've changed that. I now usually go for the loudest comfortable.
On my Android phone when I am listening for an extended period I usually put the volume just below where it warns "Listening to loud music for a long time can damage your hearing" or at a 12/15.
On my computer I usually use about 50% volume but that changes with what I'm doing (if I'm playing games online, especially shooter games, that goes down).
When amplified I really don't know. I use a Yamaha EMX mixer for my output (please don't tell me I'm doing it wrong it's all I've got).
My phone isn't sensitive enough to sense my IEMs accurately but when I put my fullsize into it I get about 85 db average. I think my Soundcraft GB8 has a meter where I can measure the throughput, then I can use the IEM specs to calculate it. Next time I bust that thing out I'll be sure to try that as I am now curious about this. Perhaps I am enjoying frying my eardrums and don't know it.