I use a Stereophile test CD with the appropriate low frequency tracks to match the level of the main loudspeakers.
I do this using the following instrument:
https://www.pce-instruments.com/us/measuring-instruments/test-meters/decibel-meter-pce-instruments-d...
You can also use this method to determine the proper subwoofer placement within the room.
This is not rocket science. There is no need to make this any more complicated than it is.
I hope this helps!