In general, I've heard it advocated to place your monitors firing down the length of the room to reduce reflections from the rear (i.e., set up against the short wall as you have done)
Support for this:
"In a room that is longer than it is wide, it's better to place the speakers near the shorter wall so they fire the long way into the room. This puts you farther from the rear wall where the low frequency peaks and nulls are most severe."
"If centered against the long wall, you are likely to sit in the node of more modes; not ideal. The reflection from the back wall will also arrive too soon if setup against the long wall."
You can always give it a try & decide which sounds best for your particular room.
The good news: a rectangle is better than a square!DO's and DON'TS of MONITOR PLACEMENT - An article by CARL TATZ:DO'S
1. Do toe in your near field monitors to a 30 degree angle
2. Do space your near-field monitors far enough apart so that the apex of your equilateral triangle is 18 inches behind your head. (recommend 67.5 inches from tweeter to tweeter)
3. Do use speaker stands and implement a decoupling element between the stands and the monitors
4. Do adjust the height of the acoustic center of the speaker (usually midway between the tweeter and midrange) to match the height of the listeners ears.
5. Do experiment by moving the engineer/monitor position back and forth along the length of the room to avoid axial mode nulls.
6. Do attenuate first reflections once you have chosen your positioning by sitting in the listening position and having someone
walk along the right side wall with a mirror until you see the left speaker. Repeat for the left wall and the ceiling as well. Place sound-absorptive panes at the wall and ceiling positions identified
7. Place absorption material on the rear wallDON'TS
1. Don't assume that your speakers are going to be truly accurate in your room, no matter how much you paid for them or how well your room is acoustically designed
2. Don't use any other angle other than 30 degrees for stereophonic monitoring. The laws of physics determined this for stereophonic listening 50 years ago.
3. Don't mount your monitors on the console. Unless your console has a lot of mass, the console resonance will greatly affect frequency response.
4. Don't use consoles with high backs that prevent proper speaker height positioning.http://www.carltatzdesign.com/index.htmlAnother really good article defining "Golden Rectangles", includes instructions & illustrations:http://www.cardas.com/roomsetup.pdf
"If you are forced to place your speakers on the long side of a symmetrical, rectangular room, create a Golden rectangle in each rear corner. Your speakers can then be placed anywhere along a line extending from the outside rear corner through the inside front corner (Diagram F)."