Bellows factor is a consideration when one focuses nearer then 8 times the focal length of the lens. (Some say 10 times). There are a number of different ways to measure the exposure adjustment for bellows factor consideration. The method that David mentioned is one of them. Another method is to measure the lens to film plane distance. In the case of a 210 mm lens for instance the infinity focus for a non telephoto lens is 8 1/4 inches. If I find that my distance is 11 inches I give one stop additional exposure. If I find that I am at a distance of 16 inches I give two stops additional exposure. If I were at 22 inches I would give three stops more exposure then the meter indicated. A good place to begin if you use this system is to type a F Stop chart as follows. 2.8, 3.5, 4.0*, 4.5, 5.0, 5.6*, 6.3, 7.1, 8.0*, 9, 10, 11*, 13, 14, 16*, 18, 20, 22*, 25, 28, 32*. The asterick stops are full stops the stops in between astericks are 1/3 stops.
By converting your lenses to equivalent inch measurements you will then be able to measure lens to film plane distance and arrive at bellows extension exposure compensation very readily. As bellows length increases above infinity focus additional exposure must be given.

Quite probably when you encounter a bellows extension situation you will also encounter reciprocity considerations. When the exposure exceeds one second reciprocity must be considered. Additional exposure must be given to adequately expose the film. When it exceed 5 seconds additional consideration must be given. As metered time increases above that additional time must be given. For instance I made an exposure the other day that the meter indicated 34 seconds exposure I gave 300 seconds to allow for reciprocity to properly expose the film. Most film manufacturers have reciprocity factors for their film. They may not be totally accurate under usage but they will be better then not factoring at all. This is due to the fact that film does not linearly expose when lengthy exposures are encountered. When we encounter reciprocity then development must be compensated by reducing development to reduce the contrast inherent in reciprocity situations. Hope that I haven't confused you but these are all considerations that you will encounter either in close up photography or in low light situations or combinations of these. Good luck