ADUHF
Headphoneus SupremusMusic And Measurements
I wonder, btw, if speakers without a rear-firing port would really need that much space from the front wall?
Sometimes moving them closer together a bit can make them image better.
It is a good starting point. The sound can be finetuned by moving the speakers and listening point. Even a couple of inches can make a difference. Stereo image can be optimized by toeing speakers in. Often the problem is not to know what to do, but the room and placement of furniture not allowing it. Often people don't like to have their speakers "in the middle of the room" far from the walls so their push them near the walls/corners and the sound is crap.
I think the rule of thirds was only intended as a rough starting point.