i may be wrong, but i think that if it depended on the room acoustics and its size and my distance from the speakers, the difference re. the normal level would always be the same, because my room and its acoustics are always the same, no variation.
let's say if my room was very live, i would have to always use lower levels so as not to be overwhelmed with harsh and muddy sounds.
as it is, my room is somewhat live in the back end, with diffusers; the whole front wall has absorbing panels and a tight weave drape over them; the left side wall in between the speaker and the MLP is also treated with sound absorbing panels and draperies. the right side is a window, so i shut down the venetian blind and there is a double layer of tightly woven drape over it. the ceiling is not treated, but its reflections are taken care of by my putting a piece of absorbing panel on top of the speaker and jutting it forward a certain distance so as to absorb whatever sound would go towards the ceiling and reflect back down to my MLP; an easy solution.
so, in effect, i don't think it's the room acoustics. if it was bad room acoustics, i would never be able to listen to music in a way that the speakers and the walls disappear in such a way that i'm transported to the hall where the sound was recorded (the recording's acoustics). this is the main thing that tells me my setup is good.
maybe if someone had a well treated room and the same films, then that person could check if the result is the same as mine: LOTR Extended Edition/Bluray: -2.5 The Godfather, the Coppola restoration/Blu-ray: +3
the majority of my movies do play as they should at the normal 0 level, like, Hanna, Saving Private Ryan, Inglourious Basterds...