Such silliness. I'd expect the best most accurate, or least inaccurate, setting is dead flat. EQs like these are hamhanded and essentially worthless. The best you can do is maybe pump up the bass if you prefer lower volumes, as the ear is less sensitive to bass at lower volumes, or pump up the treble because you know you have significant age related hearing loss.
I suppose you could play test tones with a speaker in the driver's seat and try to see what's actually going on with the stereo, but even then, the EQ is still going to be almost worthless. Now if you play audio from your phone to bluetooth and you have an app with a real EQ, then you could get more serious.
Still, better to just not touch it.