Gregory, Bellows extension must be accounted when the bellows is extended further the the infinity focus for the focal length of the lens in use.
For instance, let's take an example of using a 210 mm lens and making a close up exposure in which the bellows are extended to a length of 16 inches. The compensation method that I use is to convert the focal length of the lens in mm to inches. So in this case the 210 mm lens is 8 1/4 inches. The extension is 16 inches (measured from the center of the lens board to the center of the film holder). The bellows compensation would be 2 stops.
If the extension would have been 11 inches the bellows compensation factor would have been one stop.
Sound surprisingly like Fstops? Well, that is because it is. I have made up a table of the common Fstops, carry it with me and use it for this purpose. You can round the lens focal length in inches to the next nearest Fstop and it will be close enough for most purposes. For instance a 90 mm lens (3.54 inches) would be rounded to 4.
Good luck with 4X5. Most importantly have fun.