Two things come into play. First the angle from the seat to the L/R speakers should be between 40) ~ 60*.
Of coarse, 60* is an equilateral triangle. A 40* angle would put the seat back about 1.38 x the distance between the speakers.
The second thing is to keep the speakers as far away from the side walls as possible, at least 2ft. The greater the distance to the side walls the wider the soundstage will be, as the speaker radiating pattern will expand to its fullest, w/o the first early reflection bouncing right back into the speaker, which would muddle the sound.
Having one side open causes problems, as having a symmetrical room is best. But you'll just have to play around with speaker locations until it sounds best.