Edwin, I don't use Avia and haven't studied it so as to reply to your first question. On your second and third questions, however, don't worry about getting exactly 85dB or any other number; the main point is to have all the speakers as close to each other as possible. Avia sends out its calibration tones at 20dB lower than the 105dB maximum which movie soundtracks might theoretically hit, so 85dB is the target. Most(all?)receivers have their built-in test tones 30dB down from the maximum, so because of that lower level the target is 75 rather than 85dB.
