No formula can take all the variables into consideration. You can find some info that may give you a rough idea, but to get best results, you need to experiment. If you have a standing wave problem, its easy to hear, and you usually don't have to move the subs very far to fix it.
To prevent sub to sub cancellation, place them either less than a quarter-wavelength apart or more than two wavelengths apart for their pass band, which for 40 to 100 Hz means less than 2.8 feet or more than 56 feet.
Plus, you still have boundaries to consider. The subs should be next to the wall, or more than 8' from the wall.
You can see why coupling is easier and usually sounds better! However, as per the previous posts, the shape of the room will be a big factor, and it's usually difficult to get them 56 feet apart!! My personal best advice (from experience running subs spaced apart) is 1) use your ears and 2) do not place them symmetrically (mirrored) in the room.