I did exactly what you did a while back as well, OP.
To simulate the dB level the ears would get...
I ended up measuring the dB level by cupping my hand around the db-spl meter and even tried a cut out with index cards/cardboard to see if the readings would change. They did go up by a few dB (3-5 dB) when I cupped it so keep this in mind if you are measuring without closing the space around the db meter.
There's a fancy chart and report out there somewhere on the net and I looked at it back then. All I remember is that the consensus was to stay under 85 dB at all times. It also mentioned that hearing loss is accumulative and gradually sets in for many people when they're older. Listening to music for hours at 85 dB is still very risky since there isn't definite proof that even 85 dB is okay. It's an arbitrary line that was set for the workplace.