There are a number of programs available that will take your room dimensions and show you where the modal PEAKS are. I have yet to find one that shows where the NULLS are. Anyone know where one might be?
Nulls can be extremely tricky to identify and in the end it is really a matter of what the response tells you. One of the easiest ways to identify nulls is to have someone, preferably someone of a larger size, stand flat against the wall on each side and in the rear of the room while you run a sweep. Each time you locate a null, that is the general area the panel should be in. From there, have the same person hold a mirror in that area against the wall and place a panel at the point that you can see the speaker in the mirror from the spot that the mic/SPL is taking the measurments.
I know it sounds too simple and non-technical, but it works.