as a general set of rules... because nothing is ever equal in the real world.
I make my placement measurements CTC (center to center) or 'on axis'.
I prefer my speakers placed as wide as possible (X axis), and while MLP (main listening position... Y axis) might be somewhat foreshortened, it makes for a wider sound stage, which is what I prefer. Currently my personal speakers are 17'ctc apart, and my MLP is 16' on the Y axis.
for me personally, there are some other aspects that I also try and adhere to... I don't like Boundary gain, so I never place my speakers against a boundary layer/wall to the point where I even preferred my Klipschorns AWAY from any wall and I used 'wings' to augment LF extension. I also DO NOT like sitting up against a rear wall where reflections and time arrival issues are experienced.
If I had a small room, I would use a near field layout myself.