Much has been said on this before, though I don’t think there has been a dedicated thread, so perhaps this is the place to gather experiences.
Aside from individual manufacturers’ guidance there are various published approaches.
A popular one is the Cardas method:
There’s another approach by Arqen:
http://arqen.com/acoustics-101/room-setup-speaker-placement/
And if you are choosing a room, or building from scratch, Salford University’s Acoustics dept. Has done very useful research, with spreadsheets identifying best dimensions for rectangular rooms: links in this post: Room dimensions - #2 by Innocent_Bystander
I used REW software with a measuring microphone, playing around a theoretical ideal listening position 38% of distance from front to back wall (and trying all feasible orientations), otherwise keeping the speakers on the corners of an equilateral triangle and finding best position as I moved them closer to tge wall or closer to the listening position. Using measuring software is far more accurate than my ears, and much quicker! The result is pretty good, not far from a nice even response (now just requiring some room treatment to reduce near reflections and reduce decay time).