HDRI with Fisheye without Nodal Head (using extra long Arca Swiss plate)

This seems like it should work, but it's very difficult determining what's parallax and what's simply fisheye distortion? Anyone have any advice for finding the nodal point of a fisheye lens?

Edit: looks like fisheye lenses have nodal points dependent on angle of rotation.... this might be a no go.

Further edit: Apparently the very front of the lens (the gold ring) on the Sigma 4.5mm is approximately the nodal point. Unfortunately if you put this at the edge of the tripod, the knobs for the ball head are visible and take up a rather large portion of the frame. Another reason panoramic heads cost more. :/

Regardless, with an extra long plate, I was able to get the camera/lens within an inch of the nodal point, and have had pretty good results as it seems like that's a good compromise between correct placement as regards the nodal point and not getting the ball head knobs in frame. I'm still working on my HDRI technique but this set up seems reasonably good for my purposes. (Occasionally wanting an HDRI probe for CGI.)

Not that anyone asked, but if someone googles this... it seems like a decent low budget compromise, but it's not without its trade offs. (If you get the lens to the nodal point more exactly, you also get more of the ball head and its knobs in frame.)

Of course, using a Digital Rebel XT and this Sigma lens has not resulted in the same breathtaking 16k HDRI maps I've seen online... but then again these are of the area where I shot them and I only paid very little for the lens and extra long plate.