How to arrange my speakers?

A lot of good thoughts here. My rule of thumb is simple, based on minimizing room nodes. Begin with the speaker. You don't specify ceiling height, but likely it's either 8 or 9 ft. That makes the first order room resources at 1125/11.5 = 49, 98, 196 Hz; 1125/18.5= 30, 61, and 122 Hz and 1125/8= 70, 141, 281 Hz or 63, 125, and 250 Hz. Now comes the tricky part,you want to locate the center of the woofer where it minimizes reinforcing those frequencies while still maintaining as close to a ratio of .6:1.0:1.6 from the woofer center to the sidewalls, floor, and back wall.  Hint: no room ever allows a perfect combination of all that. If you want to visualize this, try Amroc at amcoustics.com. it will allow you to see the 'pressure points' at various frequencies. This is also helpful in understanding room treatments (panels & diffusers). BTW, your room falls within the 'Bolt Circle' of recommended room dimensions for listening spaces. That's a very good place to start from. Just remember, speaker placement is acoustics, and acoustics is physics, and physics is math. But a lot of people have built online tools to do that math for you. That leaves just the final adjustments to your ears.