I did the following to find the correct nodal point for several lenses.

I put the camera on a tripod with the offset mechanism (in my case a Kaidan VL adapter). Then I opened the camera and shutter (B and a cable release) and projected a laser beam through the lens center and onto a crosshair on paper taped to a wall behind the camera. I then adjusted the offset so that the laser stayed on the crosshairs as the camera was rotated in azimuth. Cheap, easy, accurate, and you know you're on the right node.

Hope this helps.