advice on a back focusing 17-35

hi all

today i decided to check my lenses with my new D800..and im pleased to report that the 24-70 and 70-200 were fine at all focal lengths at F2.8..my 17-35 however is great at 35mm and 24 mm..but at 17 mm it back focused quite a bit..requiring around a -20 to compensate...any ideas..I dont think you can set a FL setting in the af fine tune..can you ?

I dont think ill ever use the lens at 17 mm so close as DOF not to mask the problem ..but one never knows