From what I see in your picture there is no plane (imaginary wall) that would allow the rears to be mounted equal distance apart. If you want to include all seats in the  surround “bubble” I would suggest the blue squares. I assume that would be a wall mount at about 6 ft above floor level. The red squares either puts a speaker on a listener head (on the left) and/or excludes a listener ( the stand-alone chair)

Note: Your space is very challenging as all mounting points are off-set to each other. Running TruePlay with an iOS device may help in tuning the space.

thanks for getting back to me. it seems like the blue placement is a bit too far away and the red placement is a bit too close. are there downsides of having the rears so far away? if i do the blue placement is ear height or ceiling height with upside down placenent better? do the era 300 speakers need to be equidistant from eachother? what direction should they face?

From another perspective, I find surrounds, no matter how poorly placed sound better than no Surrounds.

Mine are over my head, three feet apart and facing out of a corner of the room at 90 degree angles. Still sounds better with them toggled on than off.

Don’t let perfection keep you from “pretty good.”