Experimentation always required. Starting position, “cross the beams” 18” behind your head in the listening position. I put a tall object behind my listening chair and use a laser measurer to point the speakers. This has the added value of checking the distances are the same.
This was the recommended position for my speakers. But I noticed the sound field was restricted… and my speakers are known to disappear. So, I gradually changed the toe in until there was none… still had a strong central image and the sound field opened up and the speakers disappeared.
You just can’t be sure until you try. More toe out until the central image starts to get weak. It is great practice because it helps demonstrate what a sound field is and how it works.
if you have a large enough room… pull the speakers out another foot. Reduce or enlarge the space between speakers. Each will show you another variable of the sound stage.