Been finding the NPP point and found it at 156mm on the upper rail for my D800 with 14-24mm on. Trouble is when I go to shoot the Zenith shot the clamp knob hits the lower rail thus not letting me shoot straight up. I do have the Nadir adaptor on to raise it up but it's a few mm not raised enough. Is there any other way? Is there a clamp with a smaller (shorter) knob? Is there a longer vertical rail, I already have the longer upper and lower rails.