I think most of us guesstimate.
If you listen to much 2 channel, especially critically, the front left and right should be further apart than 60 degrees. That will depend on your setup because you also want them as far from side walls as possible.
We all have imperfect rooms and short of learning a lot about acoustics, trial and error is usually the way to go. I'd suggest starting with getting the front left and right speakers in the best spot using music that you like. Presumably there is only one spot for the center channel so that just leaves the surrounds. If you are using regular monitor type speakers, the best spot is probably just behind but angled toward the listening position. Mine are at ear height and aimed toward my shoulders. Higher is better for movies, ear height is better for surround music.
Hope that helps.