Still getting sorted out with my new NN3 and my setup : Canon 40D with Canon 17-40 lens.

Basic Question here in aligning my Lower Rail for this set up and shooting at 17mm...
The question relates to pointing the camera down and looking through the viewfinder. In the NN3 manual it states that when pointing the camera down, you should see the NN3 knob's bullseye in the center of the frame.
I am intepreting this correctly?


On NN suggested settings (http://nodalninja.com/nodal_ninja_3_..._settings.html) it lists a suggested lower rail setting of 58mm when the lens is set to 17mm.
However, If I do this and point the camera down at the nadir, I do not see the NN3 bullseye in the center of the frame.
It seems I have to set it at ard 54 or 55mm to get the center of the frame to align with the NN3 knob's bullseye.
Note - I AM using the camera plate provided with the NN3 kit.

Please help, as I want to make sure to eliminate parralax in my panos...
Thanks