What determines speaker distance?

What determines the spacing between the speakers? I've seen a 1.3 formula for setting the distance between each. If I have a 9 foot distance from the speakers to the listening chair that would make the spacing approx 7 feet? Seems a little wide to get a good soundstage. Are there are any other "general" guidelines for getting a basic spacing between the two?
All formulas aside, where the center image starts to lose focus is the maximum spacing distance for YOUR listening space and position therein.
To add to Gs5556 comment - the further apart your speakers, in general, the larger the soundstage. Try STARTING at 7 feet and opening this distance up to at least 9 feet, until the center image loses focus.
I agree with both the previous posts and would add that you should also consider experimenting with different degrees of toe-in.

I own two pairs of ProAc monitors and their manual recommends toeing them in so they point directly at the listener so neither side of the cabinet is visible when looking at them. However, just as having the speakers closer together will help center focus so too will toe-in, but may also have the effect of narrowing the soundstage.

The set up I'm using is with the speakers six feet apart and my listening position varying between 6-8 feet from the front planes of the monitors. I have them toed in about half way between straight forward and pointing at me. Even in my small room this placement gives me the balance of focus and soundstage width I enjoy.
It all depends on your room and your speakers, but all of the above is good advice. Try checking out one of the set-up formulas (like the one on the Cardas website). They take your room dimensions into account. The speaker to speaker distance, speaker to side wall distance, speaker to rear wall distance, and speaker to listener distance are all part of a formula based on your room dimensions. Try using the formula as a starting point and then make adjustments from there. My Merlins also recommend a slight toe-in, but that would depend on your speaker's design.
Of course all this depends on your room etc., but a pretty good rule of thumb is an equilateral triangle. Maybe, you(the listener) just a tad furthur away from the speakers. If your speakers are 8 feet apart you should be at least 8 feet, perhaps, in some cases 9 feet from your speakers. Just one of those rules with a zillion exceptions. Listen and thee shall hear....warren
The formula doesn't work well in my room b/c the speakers would then be in the middle of the room, not too wife friendly. The 7-9 feet seems a little more then where I am now. I think they are around 6'6" center to center, the soundstage improves as I keep adjusting but still not there. They (Vandy 2ce sigs) also sound too harsh with more than an inch of toe-in. I didn't think they would have any center image with 8-9' ft of spacing center to center. I'll continue moving them around & see where it gets me I just didn't know where most folks are running their speakers as a general rule of thumb.
I'd say it's determined by how bad the speaker's breath is...
~~~~Clamps, got me laughing on that one!

Jest The Facts

I couldn't help myself- the question had me rolling on the floor. But seriously, What I meant to say is put 'em where they sound the best to you....
Usually, the length of your speaker cables determines the distance between the speakers.