I have been experimenting with this dimension and the distance to the side walls quite a bit lately. My room is on the small side at 17x11x8'. The speakers are on the short wall.
A good friend of mine who regularly sets up Wilson speakers for his clients visited this weekend. The Wilson method for locating speakers is to have someone stand against the back wall facing the listener (who is seated in the sweet spot) and begin talking and walking forward. When the person's voice becomes most natural sounding you stop and make a mark on the floor. The same procedure is then done from the side walls. Then the entire procedure is repeated for the other speaker. At this point you have established a rough idea of where your speakers should end up. The rest is fine tuning through listening and adjusting.
Having done this exercise, I ended up with the decappo's about 3 feet from the back walls and 20" from the side walls. More listening and adjusting ended up bringing them out 51" from the back walls and 21" from the side walls. Go figure.
Obviously each room is as different as the listener's tastes. I'm still experimenting (and probably will never stop), but I thought this might be of some interest and use to the group.
Best
Andy