There is NO set formula that works for all speakers in every room. Due to differences in reflection points, reinforcement and cancellation nodes, radiation patterns, slight irregularities of room dimensions, etc... It is all a matter of trial and error. While there are some computer simulations (CARA) that supposedly take ALL of this into consideration, i have not yet attempted to try it even though i do own it.
I would suggest running the speakers with little to no toe-in. In order to do this, you have to play with the spacing from speaker to speaker, speakers to side walls, etc... This affects the width of the soundstage and how strong the central image is. Once you get this right, you can then play with the distance from the wall behind the speakers to the speakers, etc... This affects the tonal balance due to room nodes and the overall "depth" of the soundstage. You may have to go back and forth between the front to back and side to side spacing a few times to get it as "dialed in" as possible since all of these changes interact with each other.
Rather than try to use some type of "generic" formula that does not take into account the specifics of each installation, this "fine tunes" the specific characteristics of your speakers to the specific characteristics of your room and listening position. Short of inputing TONS of very specific data into a computer, this is the only way that i have been able to achieve the results that i've been happy with. I did a post about this quite a while ago and will see if i can dig it up. If i can, i'll post a link. I think that you'll also find that putting the speakers on the long wall rather than firing into the depth of the room will help IMMENSELY. Hope this helps. Sean
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