Triangulation is a rule of thumb, and usually much better than what some might do without any guidance. But getting it right for any particular speaker is dependent on the speaker, the room dimension, room treatment, and sitting position. But as a starting point the equalateral triangle is a good start, but that sound incredibly close to the rear wall and some [?] most [?], certainly not all, speakers will have a hard time with stereo center fill if they are 12 feet apart, more than 7 with mine and the image density is reduced, though it does make for a "wide" soundstage. The rule of thumb works, but rarely ideal - it is a starting point.
