I have always wondered how effective a backrest is for the driver. I am not sure I would even be able to lean back on it since I lean forward. I am 5'11". How does it feel? Do you have stock bars? Risers? Heli bars?
When I had this on the bike I had 2" MCL risers and home made version of these:
http://www.advancedsporttouring.com/Concours_14_Risers_p/c14-16.htmSo my bars were 3" higher and 1 1/4" further back. But your right, with out risers or different bars there is no point in the back rest.
I now have Heli Bars Horizon ST bars with a Corbin seat.