This is the easiest way. Convert the focal length of the lens to an f-stop (210mm would be f8 - 8 inch lens; 150mm would be 5.6 - 6 inch lens). Okay? The progression in bellows extension is now the same as the progression in f-stops. A 6" lens (150mm) lens racked out to 8" is one stop more because f8 is one stop from than 5.6. Racked out to 11", it's two stops, etc.
Strictly speaking, this isn't scientifically accurate. The example above would be about 1/2 stop overexposed. Inconsequential. I've used this for over 20 years and it always works. You should have seen the expression from the RIT slide rule-type "teacher" when I brought this up in class in 1980. Nitwit.
Also - same size images always requre 2 stops more exposure. For a same size image, the bellows will always be extended to twice the focal length.
One more thing pleeze - Life is easier if you use short lenses for close ups.
Okay? Now go take some beautiful pictures
Anthony - Fine Art Photo Supply