Hmmm.... I'd start with the rule of thirds for both axes to get a good estimation of the total differences. Then maybe tighten up the long wall geometry to the same as the short wall one to further isolate just the ROOM difference...in other words set of a nearfield triangle and try it both directions. I use such a geometry on the short wall, creating a VERY deep sounstage (8 ft behind the speaker plane). Sidewall tuning becomes more critical, but with great speakers will yield a very wide stage, as the 60 degree or so angle with the listening chair is still sumptuous. It all depends...no easy answers here.
Just have fun, trust your ears, paying specuial attention to toe in and sidewall splatter. Some speakers--especially 3 ways or any with widely spaced drivers, will require a minimum lestener distance before cohering sufficiently.
My home demo of N803 comes to mind. They needed 10+ feet to work, as the midrange/woofer crossover is too high at 400Hz. So there's a speaker that ONLY works on the short wall in a long triangle, for example, but with special attention to sidewalls to dampen the hot tweeters' flare.
Two-ways will generally cohere well in the nearfield, giving you more room setup geometries that are acceptable tonally. As you might sense, I'm a fan of closely-matched stereo pairs in the nearfield, using a 7.5' equi triangle in a 14x24x8 room. Cheers.