I am getting a different distance for the lower arm for the Canon 40D using the supplied camera plate. The upper arm is at about the 49 mm marking. I have attempted to make the measurement with a set non digital calipers and get 51 mm +-error (I am not able to measure this very accurately) The table has a lower setting of 54.5 mm for the Canon 40D using the camera plate. I am looking through the view finder with the upper arm at 90 degrees and the tripod leveled using the bubble in the NN5L. I also switched on live view and changed to x10 setting to see if they agree. The middle exposure point is shifted slightly to the right

I have been using my NN5L for a while with my Canon 40D and Sigma 10mm fisheye which has a 167 degree on Canon APS-C and 180 degree FOV on Nikon DX APS-C sensors. Initially I did a quick alignment with a Zantac pill bottle and a pen. I also shot with too many stops at eight detents. I am now doing what I should have done initially and fine tuning my settings. I am going to change to six detents and see how that effects my results.

I will be calling Sigma again when a particular person is to see how different the entry pupil site is on the 10mm versus the 8mm.
Focal length, diameter (mm), length (mm), groups, elements
8 mm, 73.5 mm, 68.6 mm, 6 groups, 11 elements
10mm, 75.5 mm, 83.1 mm, 7 groups, 12 elements
http://sigmaphoto.com/lenses/lenses_chart.asp

EDIT 04Aug2008 10:56am pacific:
Just rcvd information from Sigma USA support
This information is measured from image plane when lens is focused at infinity (elements move when focusing) and something about Lens head (lens head term may have translation issue)
Lens, 1st Nodal Point, 2nd Nodal point (rear, defines focal length), Entry pupil, Exit pupil
Sigma 8mm f 3.5, NP1 87.4 mm, NP2 8.1 mm, Entry 94.3 mm, Exit 54.0 mm
Sigma 10mm f 3.5, NP1 88.8 mm, NP2 10.2 mm, Entry 97.5 mm, Exit 72.5 mm

Tangent: Sigma support said diffraction losses start at lower aperture for wide angle lenses. He says that I need to try different stops empirically and see where I get better results. I have usually been shooting at f11 because I read that that was a sweet spot for the 20D and a particular lens for depth of field and diffraction loss. Of course this is with the caveat that you don't want to shoot very open with a wide angle lens unless necessary to get the shot (low aperture)