On any NON PANEL speaker (A speaker in a box) You will find that the closer the speaker is to a back wall, the more bass reinforcement you will add. The closer your are to a side wall, the more reflective frequencies that you have to deal with. On side walls, the more tow in that you have the closer you can get to side walls without huge issues.
So start with your your room size and how far your seating position is from the back wall...
In general, I have always been able to obtain very good speaker placement. Start with measuring how far your seating position is in front of how far you would consider where you would want your speakers pulled out into the room. So, pull your speakers out to where you believe they should be... hopefully a minimum of a few feet from a back wall. Measure now from your speakers to about where your head placement would be if you were sitting in listening position. If this is 6 feet, try to get a 6 foot distance between your left speaker driver and your right speaker driver... if it is 8 ft from your head, try to get a distance of 8ft between your left and right drivers. This does not need to be exact, but in general works well. Now that they are seperated left from right, how far are they from the side walls?, try to ensure a minimum of 2 ft, 30 inches is better. Now, tow your speakers in to point at your ears when in your seating position. This should give you pin point imaging... if you have a hot spot or your speakers are a bit bright, start towing them outward, just an inch or so at a time. With some speakers, you can tow them outward until they are straight forward front firing, others you will destroy your stage with just a few inches.
The close that you get to a full front fire, the more the side walls will cause reflect and cause frequeny peaks where you will need to deal with room treatments or pull your speakers in more to the middle of the room ... 36 inches, 40 inches etc.
This method has proven very good for me. I hope that i've explained it well, good luck, Tim