There are a few things you need to consider:
- symmetry is critical for a good stereo image - you seem to have that pretty well settled
- Placement of your loudspeakers and ears is critical - and there are no secret formulae for that, you'll need to experiment.
- you want as much distance as possible between your ears and the back wall - up to about 12 feet, so facing the short wall, in your case, will be better.
- different loudspeakers have different requirements with respect to placement - refer to the user manual or web site.
Auralex is not in the business of giving bad advice, but you also have to remember that their blueprint is free, and sometimes you get what you pay for.
It is possible to design a studio on paper alone, but it requires experience (which arguably they have), and time (a LOT of time), and some tools or great math skills. A long time ago they employed a brilliant studio acoustician. He has since moved on, and I don't know who is doing their design work, but I would take it with a grain of salt. But then I'd take advice from almost any acoustician (Russ Berger, Wes Lashot, and George Ausperger come to mind) with a grain of salt<G>!
It isn't always practical, or financially feasible, but even a couple hours of a good designer's time could save you a lot of money and frustration, especially in a smaller space.
All of which may sound terribly negative, and it isn't. One of the best control rooms I ever worked in was oriented along the long wall, with a huge window into the studio, and two grown men had a difficult time passing behind the console. Yet every track that came out of there translated beautifully. A big part of that, of course, is that the studio owner knew the room, but even stuff I mixed there worked well. The studio was designed by a gentleman who spent his early years designing studios in Europe, where small control rooms were the norm. Clearly he learned his craft!