On Denon AVRs ... the "absolute scale" goes from -88.5db to +18db with 0db being reference volume, while the "relative scale" goes from 0 - 99 with 82 being reference volume. Likely the same on the Onkyo as well.
After I calibrated my Sc-27, I listen at -25 to -20 (rarely). Am I a girlie-man?
I'm really not sure if MCACC sets ref level to 0 or not. On my Denon, I usually watch TV around -25 or so but I crank movies and music to around -10 to -5. It gets extremely loud at -5 but it sounds good for action movies.