If your sub is on the same end of the room, near your mains, it will most likely be 0. If it isn't obvious by listening, you would probably need a SPL meter to tell whether one of the other settings would be better, but again, if you can't hear it, it probably is fine at 0 as most systems typically are. If your sub was on the opposite side of the room, a different setting (180) might work better.