How do you find your safe volume level?

[MadGman]

At what point music is too loud?

Today, while driving a car I had a little quarrel witch my brother about the loundness of music in my headphones. He says that if he can hear my music when I take headphones off my head, I listen too loud and it can damage my hearing. I disagree. I’m driving M40x from my phone. I need other people opinion to show him I am right.