Quote Originally Posted by jeffreyg View Post
My understanding is that published filter factors are not always correct and can vary from mfg to mfg as well as with the color of the filter and the colors of the subject. It would be best to meter through the filter and without the filter to see the difference before making the adjustment if that degree of accuracy is important.

http://www.jeffreyglasser.com/
This will work only if the meter cell and the film have identical spectral response, which is not often the case.