This guy always made a lot of sense to me. I'm just throwing it out there as additional information.....
http://www.acousticfields.com/speaker-placement-listening-positions/
http://www.acousticfields.com/acoustic-rule-2-speakerlistening-position/
http://www.acousticfields.com/speaker-placement-listening-positions/
http://www.acousticfields.com/acoustic-rule-2-speakerlistening-position/