For me personally, I connect my phone to my car and always have my phone's volume at 100% for the Bluetooth because I control the volume with the physical knob in the car.
Man, I hate that BlueTooth doesn't have an equivalent of "line-out" that isn't affected by the host devices' volume settings. It's so annoying when I can barely hear my music because I turned the volume way down on my phone while watching a video late last night.
Poweramp plus allows you to set audio profiles for different devices, I have never gotten it to work properly between my bluetooth, wired headphones, and android auto.
It's should to be close enough, the spec is called sensitivity (SPL) and most headphone manufacturers try to hit around 100dB/mW.
Hopefully the setting would allow you to fine tune it based on what headphones you have.
Samsung turns the volume icons green beyond 60%, and it's much better than nothing; I would've raised the volume way above that way too often, if it weren't for that feature.
There's a feature to limit increasing the volume beyond some point, which—if you enable—you'd have to disable it to increase the volume, but I find it unnecessary.