Determining reflection points

Mrjstark is correct for most speakers, and correct about the mirror technique.

Note that line-source speakers don't have a significant floor and ceiling first reflection. Note that bipoles and dipoles will have a significant first reflection off of the wall behind the speaker. And if the speakers have a well-controlled radiation pattern, it's possible that enough toe-in could essentially avoid the first near-side-wall reflection.

Duke

Thank you Mrjstark. That makes it easier.
And thank you Duke. That may make it even easier. As always Duke, you rock!

YES "Audiokinesis", dipoles are exception ( rear wall seams to play far more important role in setting up those kind of speakers then reflection points on side walls). Speakers with very small radiation pattern are indeed less affected by side walls sound reflecting surface. Like you said , with careful placement and toe-in side reflections can be greatly reduced.
Good example would be a pair of Martin Logan speakers. They are dipoles and have radiation of about 30 degree horizontal and almost non vertical. Initially, someone may think that Martin Logans have to be easy to setup and are room friendly ( side reflections play very little factor with ML ). That someone could not be farther from the truth. In fact those speakers are difficult and tricky. However when setup is optimize they can be very rewarding ( got'em for 6 years ).

Also:
Using the mirror technique where you see the other speaker's tweeter on the side walls.
Where each speaker's tweeter bounces of the rear wall to your ears at listening position

See: http://www.realtraps.com/art_room-setup.htm)

I don't think this will answer all your questions, but the diagrams should answer some.