I set up a very cool little box, with a screwdriver poking out the top of the box, with a slit cut into a piece of cardboard that was placed behind the screwdriver. Using this setup I was very easily able to find the nodal points of my shorter lenses.

With my Canon 70-200, however, I found it next to impossible to figure it out at the long (135-200) end. Even looking through the viewfinder, as you move your head slightly (without moving the camera) you can see some parallax. I ended up just sort of "guessing" a value.

Any pointers for this lens? I have it mounted up using the tripod collar.