First, you're blessed to have such a large room, few have that. Second, spacing depends to some degree on how the panels are built. If they are open backed, there is an advantage to leaving a few inches of gap behind or above them. An open backed panel allows sound to go through and bounce back from the wall/ceiling behind it and come through a second time. Acoustically it behaves as though the panel is deeper, which is usually a good thing for lower frequencies. A closed back panel is fine right against the wall or ceiling.
If you're going to do room treatment, you'll get the best bang-for-the-buck by straddling corners, either wall/wall or wall/ceiling, with open backed panels. Sound tends to collect in corners and be reflected back into the room in many directions, not good.
Also, by straddling corners you again allow the panel to acoustically behave as though it is physically deeper and can therefore have more effect on lower frequencies.
Treat your first refection points first and then as many corners, wall/wall, wall/ceiling as you can.
Don't know how knowledgeable your are as far as acoustics, so forgive me if I'm saying things that you already know...........Lower frequencies have very long wave lengths, many feet. As the frequencies get higher, the wave lengths get progressively shorter. For instance, the wave length for a 100 Hz signal is ten times as long as it would be for a 1 KHz signal. That makes low frequencies difficult to treat and higher frequencies fairly easy.
What I found in my own set up was that by straddling most of the room corners with 3 inch panels I was able to make considerable improvements in the bass response, as well as everything else, without actually building dedicated bass traps,.
Hope this helps.........it made a very substantial improvement in my own system, for a fairly small amount of money.
