While most posts answering your question suggest pulling your speakers several feet off the rear wall, Focal, who know something about speakers, do not ascribe to this theory. Focal uses a flexible equation to find the best speaker placement.

“Optimisation For perfectionists, here is a formula for optimal positioning: If A is the distance from the centre of the woofer to the nearest floor or wall, B is the distance to the next closest floor or wall, and C is the greatest distance (A < B < C), the equation B2 = AC defines the ideal loudspeaker position. • Example: If the centre of the woofer is 20” (50cm) away from the rear wall (A) and 24” (60cm) above the floor (B), then the side wall will be ideally 28” (72cm) away [C = B2 ⁄ A = 28”

I found this a little confusing at first, but it became clear in short order. There are three distances involved. Floor to center of woofer, Side Wall to center of woofer and rear wall to canter of woofer. The shortest distance is always “A”. This is usually floor to center which is the only constant. The next shortest distance is always “B”, and the longest distance is always “C”. The distance “C” is always (B x B) divided by A.

By example, if your floor to woofer is 24 inches, and you are 6 feet off the back wall, you would need to be 42 inches off the sides wall. If you wanted 8 feet center to center on your speakers, your room would need to be 15 feet wide.

In a nutshell, Focal suggests there is no rule about distance from back wall as long as C = (B x B) / A