How to find the nodal points is described here. But keep in mind the nominal focal lenght, e. g. 90mm isn't always the effective focal lenght. With the Super-Angulon 90mm f/8 it's 90.7mm +/- 1 %. This datas one can find normaly at the lensmaker's website.
Originally Posted by BetterSense
For LF practice it's easier to calculate with the distance between the front of the lensboard and the film surface focussed at infinity. For the SA 90mm it's 99.4 mm. Now you can calculate sufficient all you need as a photographer. Like
magnification = focal length / (subject distance - focal lenght)
close-up exposure time = (magnification + 1)^2 * time of distant exposure
and many more