to calculate easily get a small tape measure:
Measure the bellows extension from lens board to image plane(ground glass).
Take the focal length of your lens in inches and the extension of your bellows in inches. ie 210mm lens = 8" lens roughly.
bellows extension of 16"or 16.5".
Now using the 8" and 16" measurements think of the 8 and 16 as F-stops.
So the difference between F8 and F16 is 2 stops and that is your compensation.
This is most easily calculated or taken care of by changing your ISO down this amount or in our example, 2 full film speeds. So if using ISO 100 you would simply change your ISO to 25 and you have your answer.
Takes much less time to do than to tell you.