Tboooe,
I tried 2", 4" and 6" 2'x4' Acoutimac panels at my first reflection points in my 10' x 12.5' x 8' room.
The 6" was too much, the 2" was too little, the 4" was 'just right'.
I also found that I need carpet on the floor at the first reflection points on the floor.
You'll be surprised at the clarity you'll gain.
Chuck