A quick way to get the corrected f stop for bellows extension for all formats is to convert the focal length in mm to inches by dividing by 25.4.
If you then make up a list of F stop numbers as follows:
2.8* 3.0 3.5 4* 4.5 5 5.6* 6.3 7.1 8* 9 10 11* 13 14 16* 18 20 22* 25 28 32*
The numbers with asteriks are whole stops. These conversions of lens mm to inches can be rounded to nearest corresponding stop.
If we were to use a 75 mm (2.95 inch) lens on the Bronica and add the 28 mm extension tube to come to an actual 4.05 inch lens to film plane distance the exposure increase would be 2/3 stop increase in exposure.
With a 150 (6 inch) mm lens on the Bronica adding the 28 mm extension tube would give the lens an effective 178mm (7 inch) lens to film plane distance. The exposure compensation would be an additional 1/3 stop.
In the case of a 210 (8.25 inches) Symmar lens with a lens to film plane distance measurement of 300 (12 inches) would require a 1 and 1/3 stop increase in exposure.
In another case of a 90 mm Super Angulon (3.54 inches) with a actual lens to film plane distance of 120 (4.72 inches) would require slightly more then 2/3 stop of increase in exposure.