"Experiment!"
There are many "theoretically correct" ways going about it.
But, the problem is every room "sounds" different!
1. A room is nothing but an enclosure. The dimensions of the room may not allow us to follow these principles. Some rooms are longish while some are broader and some not symmetric.
2. There will be different objects in your room that will act as reflectors and absorbers/dampers.
3. Practicality: Many times we need to compromise with just being practical. The best sound my speakers gave after experimenting was keeping them almost 1/4th way in the middle of the living room! Needless to say my audio nirvana lasted on that day and was back to original placement the next morning
For experimenting, try following:
1. Start with a minimum distance of say around 7-8 feet between the speakers. (If your room permits)
2. Leave some space behind the speakers (keep them couple feet from the back wall)
3. Play some music and stand between the two speakers - starting very close to the speakers. Now start moving away from the speakers. Observe the sound stage, etc changing. Keep moving and you will notice that at a point you get best results. (This is usually near the equilateral triangle formed by user and the two speakers)
4. Now experiment with the listening "height". Try to keep your ears at tweeter level of speakers.
5. Experiment with speaker "tow-in". Try to make the speakers face slightly inwards and see how it affects the sound.
6. Feel free to experiment and fine tune and tweak all above parameters. You will find that one combination will works the best than others.
I think this tweaking is very important in any setup to get the maximum sound. You may be really surprised at how different/better the same system will sound with a little tweaking! Try it for yourself and share your experience
Cheers!