The most critical setup dimension, in my opinion, is the distance between the back of the panel and the wall behind the speaker. In general, the more the better; you are adding delay to the arrival of the backwave energy, and the more the better. If you can do 7 feet, great. In my opinion 3.5 feet is the minimum; less than that and you probably should get the Sallie or make your own equivalent.
I used fake ficus trees to diffuse the backwave energy at the first reflection zones on the wall behind the speaker. This is where you could place mirrors and see the back of either panel from the listening position.
Distance to the sidewalls is up to you, but I'd say nine feet between the panels (roughly eleven feet center-to-center) is a decent starting point. They can go within inches of the sidewalls if needed.
Now I usually like some diffusion at the first sidewall reflection zones as well, those being where a mirror up against the side wall would show a reflection of the front of the panel from the listening position.
As for toe-in, the nice thing about SoundLabs is the tonal balance stays pretty much the same across whatever arc the speakers cover (60 degrees for the M-3). So playing with the toe-in is adjusting the relative amounts of energy going toward the side walls vs towards the middle, without adjusting the tonal balance. If a fairly strong toe-in works for you, that would probably eliminate any need for diffusion on the side walls.
Finally, measure the distance from panel to center of the sweet spot to within 1/8", and from there you can fine-tune by ear. Yup, there is an improvement in imaging from being that exact. Eyeball the panels to make sure they are both at the same vertical angle, and fix it with shims under the feet of one of them if necessary.
The above is for a "conventional" setup. Feel free to try whatever unconventional setup you feel inspired to; SoundLabs are a lot more flexible than most people realize. I had one customer who set his SoundLabs up in a "nearfield" configuration, so close to his listening chair that he could lean forward and touch them. Obviously this was a one-person setup, but it was pretty amazing for that one person.
Finally, feel free to e-mail me if you have further questions.
Best wishes,
Duke LeJeune
dealer/manufacturer/longtime SoundLab guy