bellows extension factor (BEF):

BEF = bellows extension ^2 / focal length ^2
(determine bellows extension by measuring the distance from the lens plane to the film plane when the distance is forward of infinity focus)

Ex:
My 210 mm lens is at infinity when the lens and film planes are separated by 8.25" (210 mm / 25.4 mm per inch = 8.25") . If after focusing the lens is now at 10.25", then:

BEF = (10.25^2 / 8.25^2)
BEF = 1.5x