Thank you for your interest, I hope you found it useful.
The centre of the rotator is of no interest in setting up the NPP in-fact doing so can confuse the stitcher. In the end you are going to either cap the tripod nadir area or patch it away with View Point Correction or similar.
The object in setting up the NPP requires you to just capture the edge of the panohead so when stitched you can plainly see a round circle. This is typically -15 degrees with the common camera/full frame fisheye combinations.
When it comes to shooting panoramas (after you have found the NPP and OSP) I suggest you only tilt down -10 degrees as capturing the panohead is pointless and not required.
A common shooting sequence would be 6 rotational shots at -10 degrees. 1x Zenith shot at +70 degrees or 2x Zenith shots 180 degrees apposed at +60 - +70 degrees and than a hand held or tilted tripod to capture the Nadir area where the tripod was positioned for the rotational shots.