A laser can be an indispensable measuring tool. Make sure both speakers are the same distance from side and rear walls (if this is feasible). Toe in can be measured also. Measure from both sides of the tops of speakers to the ceiling; how else do you know if your floor is level (or your ceiling is not!) A test of your ability to discriminate: what is the smallest movement which produces a change in what you are hearing?
Happy Accident
I've always read that the distance between the speakers should (more or less) equal the distance from each speaker to the listening chair.
Our system is in the living room and due to furniture location, I'm guessing the former distance has always been about 10% less than the latter (math is not my strong point). Experimenting with toe-in has resulted in either a strong center image with a narrower sound-stage or vice versa. I've assumed the only way to improve matters would be to move my listening chair forward or the speakers further apart, neither of which is possible.
But, as my wife will tell you, audio brings out my OCD tendencies. A couple days ago, I was once again messing around and tried toeing-in the speakers so their focus converged about 18 inches behind my head. This meant that the distance between speakers was, as always, 9 feet and the new distance between drivers and their point of convergence was 12 feet.
As a result, the sound-stage now extends significantly beyond the speakers on each side without the center image collapsing. Actually, it's harder to detect a "center image", as the "image" spreads right across the sound-stage. The sound-stage is also deeper and resolution and bass actually seem to have improved. The point of convergence is only inches in front of a large, Craftsman style book-case with the typical "lattice" design of glass and oak. The speakers flank a slate hearth.
Lacking the sort of experience/knowledge that might explain why "breaking the rules" has paid off to such an extent, this scenario falls into the category of "happy accident". I can only guess that it must have something to do with the room.
Perhaps wiser individuals could offer a more scientific explanation...