No simple answer, it all gets much more complex when supplementing the rear wave. Four to five feet behind the far tweeter separates the direct front wave from the reflected back wave by around 4 milliseconds which is enough to perceive the rear wave as a reflection, rather than conflating it with the front wave to be interpreted as smear.
When space is insufficient such as the new lab I’m building in the basement, I am resorting to a diamond rather than a square room. By turning the axis 45 degrees, the return paths of the upper frequencies becomes much longer, and the wall bounce spreads out in time. You also get more distance behind the chair. In my case of a concrete corner vs 2 stud walls, We gain launch symmetry. The distance requirement behind the speakers becomes nearly moot due to the new reflection geometry. It solves many pesky problems if you can make the diamond layout work in your life.
Back to square launch, the right distance is when the mud of multi point launch becomes articulated clarity of separate primary and reflected signals. It’s obvious when it happens.
