Here is a simple way to calculate compensation requiring only an inch tape measure.
take the focal length of your lens in inches and the extension of the bellows in inches and using these numbers change the unit to Fstops.
What is the difference? that is the number of stops you must compensate.
example a 210mm is approximately an 8" lens and with a 16" extension you derive f8 and f16. What is the difference between F8 and F16? 2 stops!
You're done. BTW, wide angles are terrible for closeup due to excessive lateral chromatic aberration. Questions see RodKlukas.com
Should help you all. 75mm= 3"= F2.8 1/3, 90mm = 3.5"=F3.5(2.8 2/3), 135mm = 5.6"=F5.6(close enough) etc