Its the entrance pupil location you want, not the nodal point.  For the
110mm it is probably within a few millimeters of the iris diaphragm.
If the 180mm is a true telephoto the entrance pupil will be fairly far
back into the lens.  Some of my 35mm telephotos have an entrance pupil
location *behind* the image plane, but I doubt this will be the case
for your 180.

The best thing to do is to use the parallax method to measure the
entrance pupil location for yourself.

Brian
www.cladwellphotographic.com