Speaker distance setting on Denon 5800  

I got a Denon AVR-5800 for Christmas. It sounds great in my 5.1 setup but I'm interested in using the extra 2 channels. All the info I've read on surround back channel placement suggests putting them behind the main listening position. Unfortunately my couch is up against the back wall of the room so I'll have to put them directly above my head. In the 5800 setup menu should I set the surround back channel distance to how high above my head the speakers are or should I set it to 0" since there not even a foot behind me? I hope this makes sense....LOLOL