Speaker separation versus listening distance??

The usual formula is to create an equilateral triangle between the three, some sit a bit farther back.

There are many formulas however and your ears are the best judge.

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.