The Cardas guide to speaker placement is based on multipliers of the rooms width. Once the speakers are placed away from front and side walls according to the formula, form an equilateral triangle where, when seated, your ears are at the same distance from each speaker as the center of the speaker cones are from each other.
http://www.cardas.com/room_setup_main.php
If you really want to dial everything in to your preferences, follow the instructions in this video: