I have the identical issue. Did you ever work it out?
FWIW, I counted 40 notches on the main out control. The range is -80db to +10db, so a total range of 90Db
That means each notch is worth 2.25DB putting unity gain at 35 notches.
I think i’ll Calibrate mine with the main out knob at 12 o’clock and see how that goes...