Comparison of methods for determining the exposure correction required for magnification changes using a 50mm lens, Beseler 45 enlarger and condenser lamphousing
The ruler and the magnification formula provide the same result - the previous difugalty was due to not correctly locating the lens nodal point. In this case the point is 1.1" back from the front flange of a 50mm f2.8 El-Nikkor, FWIW.
However, although they agree, they both provide the wrong exposure correction - enough to create a shift of about 1/3 of a Zone in some cases. Intriguingly the error doesn't seem to follow any sort of pattern.
The error doesn't seem to be due to the variation in the distance from the lens to the condenser. Changing the distance by 1", a bit more than the lens travel in the above test, changes the light intensity by less than 0.005 stops. It might be interesting to repeat the test with a diffuse light source.
So far, it seems only a good meter will provide accurate compensation.