Why do you limit your observations to depth of center image? Why not depth of image at the extreme left and right? That's important too, no? What about basic speaker positioning issue of near field or distant/mid range. That too makes a big difference, no?
IMHO while speaker positioning, i.e. distance from walls and associated nulls, nodes, and reflections and their treatment are important, the most important ones are 1) a recording with the appropriate information, 2) the source electronics ability to pass this information through with minimal deviation from the recording, 3) and the electronics that drive the speakers (amp and pre amp) which have the ability to reproduce the most important frequencies to the reproduction of imaging with out distortion or emphasis.
If you have THE recording, source, electronics and speakers then, and IMHO only then, can your worry too much about room set up IF your goal is to optimize imaging.
FWIW I've seen too many folks try to obtain some idealized imaging without even having a source/recording which can get them there (and there really are not that many extant). If you want to chase this holy grail down get "Depth of Image" by Opus 3 (long OOP its on a CD but the LP is better by far).