Have you seen the "Bellows Correction with Bacon" video yet?
Originally Posted by Terry Bowyer
To sum it up, think of it this way. In a macro situation, you are radically changing the focal plane of the lens, such that you end up firing a good deal of the light gathered by the lens into the bellows, rather than onto the focal plane.
With a view camera it can be done fairly easy with some simple tools. You just need an object of known size, then measure the size that that object apears to be on the ground glass. From those two measurments you can calculate out how many stops to open up. If you want, we could get into the specifics of the math.