If the impedance, power out to the headphones. and the entered headphone sensitivity value are correct, it should be accurate. It’s just math once you have those figures. I’ve got a spreadsheet that does the same thing.

You can also pretty easily measure headphones with a $20 decibelmeter and a bit of cardboard, see oratory’s explanations in the comments here https://www.reddit.com/r/headphones/comments/humrj8/headphones_earphones_damaging_my_hearing/

Ive done the cardboard/meter thing with my hd6xx and it tracks what the calculations say. So now I just treat to the voltage output from my qudelix 5k as the actual volume level.

If your headphone sensitivity is correct and if this device can show accurate readouts, all data checks out.

P=I^(2)R, 0,0030625 mW=(7mV)^(2)/16Ohms, log(0,003mW)*10=-25,228 dB from 80 over 1mW and that is roughly 54,77 dB

And according to specs with 16ohm and 80mW fiio btr3k can deliver 50mW over single ended so your max volume should be around 97 dB

They’re using the balanced port for the calculation so if you’re just using 3.5mm it won’t be accurate at all.