How to have 1Vrms output using Windows?

I am using my TB with my Win10 system and I’ve installed TUSBAudio driver which solved one of my issues playing audio in WASAPI exclusive mode while watching videos (the default Windows USB Audio 2.0 drivers caused videos to stutter).

I am here to ask you about the digital volume setting to use to get a 1Vrms with a 0dBFS signal.
I am told that I need a -6dB volume (because of the 2Vrms output of the Khadas TB at 0dBFS) and I am wondering if the MASTER volume slider in the TUSBAudio control panel is using a decibel scale (which is from 0 to -127) or it is another scale and in this last case, what’s the value to achieve the 1Vrms at 0dBFS?

Thank you for your help.