I can't believe Sebrof was the first to mention toe in.
Badly written speaker manuals for the last 30 years or so have often referenced the "hole in the middle" of the stereo image when speakers get pushed too far apart.
Greater degrees of toe in will increase the distance where this first occurs, but may also change the tonal balance of the speaker.
I have also found that there is not necessarily a point of apples to apples equilibrium with greater toe in and greater distance vs lesser toe in and lesser distance.
Although there may be a relationship with respect to where the hole in the middle begins, the two different set ups can create a completely different sound and image.
Planars behave differently from other speakers, but after years of experimenting with different models, I found that setting up Magnepans with no toe in often created the widest and most expansive image, while also softening the treble a bit.
In summary, there is no simple formula or easy answer to this question and every inch counts, although is it usually a safe bet that farther out from both rear and side walls is a good thing.
