I don't think anyone actually uses those guidelines since placement is determined by what sounds the best. Every room is different and modifies the sound in a different way.Can someone help me out with my mathematical skills?
Generally you try and keep the mains away from the rear and side walls as far as reasonably possible. (~1-2 feet).
The pointing angle is then determined by how wide the soundstage is. Every speaker and room is difference when determining soundstage. Play some different stereo material you like and ensure an even soundstage that isn't too collapsed to the center and not too wide as to allow locating of each speaker..