How far apart do you position your speakers ?

Same / similar situation to Dave.
9 sided, completely asymmetric listening space. My panels are on 1 of the short walls, spaced 30->35" from the 'front' wall and about 6.5' apart, inside edge to inside edge. RH speaker has no wall near the side while the LH speaker loads into a corner....about 3' away. The sub is located near this corner, too. The ceiling is 'vaulted and about 11' high at the peak. 2 walls are at 45 degrees. The short wall opposite the speakers....maybe 24' away has a small tapestry hanging on it to diffuse and eliminate a slap-back echo I had when doing the initial installation about 24 years ago. OUCH!

A real mess, and no 'method' I can think of but trial and error works.

But accommodations have been made. I listen to the 'wrong' side of my panels and with the tweeters 'in' everything has gell'd. Bass is pretty uniform and the only problem is in the computer room....which is real boomy.

Hard surfaces reflect, corners or any angles amplify. The cardas measurements are really there with these things in mind. If you are 30 to 40 inches from any wall you are normally ok, less than that and you have a risk of bass being boosted or if firing forward, reflections. With a tow, this reflections are normally not a problem.
Corners on subs are normally not a problem if crossed very low... 40 or 50hz, you can normally tweek things with phase, volume and frequency controls, when you get near 60, this is an natural room amplifying frequency and above that is much harder to deal with.
I hope all of this helps, Tim

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The 83% isn't a rule.

It's simply an observation made after having done hundreds of successful installations around North America and at shows.

If x is the distance from your ear to the tweeter plane (our source of directionality), then y (in this case 83%) is the distance from the center of one tweeter to the other.

When speakers are too far apart, they can begin to sound too thin (although they image great). But do great-imaging-but-slightly-thin sounding systems involve you in the music - as opposed to the the sound?

Too close together, and the imaging suffers excessively.

The last system I voiced ended up at 88%. The distance is ALWAYS arrived at by listening, and sometimes we measure just to see how it turned out. But I've never been personally engaged by music that's harmonically threadbare, as opposed to rich.

Toe-in also effects that separation number, IMO, as well the dispersion of the loudspeaker, not to mention first reflection issues.

I often suggest a starting point somewhere between 80-85 (and up to 90) percent. But always, only after finding the best listening seat location ("best" meaning smoothest bass).

An equilateral triangle (y = 100%) or greater can yield uncanny imaging. But, IMO, a boring musical experience.
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If you know what your speakers 30 Degree & 60 Degree off axis response curves look like, you will have a good idea if tow in is good for your speaker. Many, Many speakers have a high end rise on axis and at 30 degrees of axis are very smooth, these speakers typically will image on or off axis, but are much more musical when facing forward (off axis). Some speakers are capable of smooth on or off axis response and sound good either way. A speaker faced directly toward you (towed in) would not sound smooth and natural if had any frequency dips or peaks. There are other factors, but this is a major factor of tonal balance and imaging when listening on axis of off axis.

If you move speakers away from walls, dampen surfaces, use tube traps or something in corners for standing wave issues, and have silent heating and/or cooling and no furniture, you can't live in my house. Period. I think the sound of a room can add life and character to systems, and other than things rattling from a sub (I hate that), I think people get too crazy about this stuff. All speakers image differently and the 83% thing seems just silly, unless you're utterly clueless about listening or you review speakers for a living. Good gear can be enjoyed AND fit in a living space without sacrificing esthetics or removing the fireplace with a jackhammer. Put the speakers in front of you and toe 'em or not until they seem to sound good on most stuff, get a beer, relax, and enjoy. This method works 100% of the time.