
Basically, bellows factor is (bellows extension/lens focal length) squared. The resulting number is your factor. You then multiply your metered time by that factor to arrive at your "corrected" exposure.
For an 8 inch lens, focused to 12 inches you arrive at 12/8=1.5, 1.5 squared=about 2, so you'd double the time. You could also keep the time the same and open up 1 stop.
Going back to an 8 inch lens, 16 inches becomes 4x, 24 inches= 9x, and 32=16x. You can see the exponential increase in factor is related to the linear increase in length. 2x the length is 4x the time, 3x the length is 9x the time, and so on.
I hope this helps and isn't too far off. It's always worked for me, although I don't often do too much close up focusing.