I would have to agree.Starting of with the equal distance idea is a good place to start.If you have 10'between speakers ,then use 10' of distance to you.This is just to start out with, although a lot of people use this in their final setup.
I guess you could use some kind of formula as a starting point but my advice is to put them where they sound good to you.After all,you are the one that is listening to them not some geek in a lab.Just a thought.
+1 on both preceding comments and consider as well that angle (toe in or out) can be non standard as well. I have found that my room requires on speaker be more than 5 degrees different from the other to center the sound image.
What are your speakers? Some speakers have different requirements than others. Vandersteens don't like side walls. Maggies don't mind side walls. Tall speakers like Dunlavys need a little more distance from the listener to integrate the drivers.