Distance to the back wall is part of the solution. You should also experiment - first start out by:
- Place the left and right speaker the same distance from the side walls
- Place the speakers from each other the same distance as your listening position to the speakers
- Make sure that the distance to the side walls are greater than the distance from the back wall, i.e. better to have the speakers 2 feet from the back wall and 3 feet from the sides rather than 3 feet back wall/3 feet side. This should determine the furthest distance out from the back wall
- Place the left and right speaker the same distance from the side walls
- Place the speakers from each other the same distance as your listening position to the speakers
- Make sure that the distance to the side walls are greater than the distance from the back wall, i.e. better to have the speakers 2 feet from the back wall and 3 feet from the sides rather than 3 feet back wall/3 feet side. This should determine the furthest distance out from the back wall