It depends on the lighting conditions of your room. If you play/watch in a pitch, or near pitch black room with little to no light sources (as I do), use 2.4. If you're in a dim, but not blackout room, use 2.2. If you're in a room where light sources and/or sunshine reflects off the screen, set it even higher. Xbox suggests 2.2, but it is taking into account how most people view their TVs, which is in the evening with lamps on and such. So, few light sources, but not none.
