Usually in x..x.2 I would put them somewhere between the front edge of the couch, and where you’re feet would be if you were seated, and stuck your feet out.
This is the “generic” image from Dolby. Since your against the back wall, I would put them just a little farther forward as I mentioned to give a little more separation between the surrounds, and heights. Another option for them is the front height position on the front wall, where it meets the ceiling.
Ime the ceiling is better, but if you or your wife aren’t excited about cutting the ceiling up, front height can work. Part of the problem though, is with any .2 height install, there’s a hole in the sound above/behind.
So maybe in your room, I’d put the surrounds a couple feet above seated ear height, and the tops at 30° foreword of the LP.