What you might want to do is to model the room matehmatically, to get an idea of what you're working with. Then maybe once you get close and have everything built and installed, you can make measurements at several positions to fine tune the installation.Check out Loudspeakers and Rooms, Part 3. It goes into the treatment of room modes in some detail. The Room Mode Calculator mentioned on page 11 is available here. The article called Loudspeakers and Rooms - Working Together is another good article. It has pretty much the same infromation, but it's presented a little bit differently.