Michael R is correct - you seem to be confusing exposure change with density change in processed film. 0.30 is the logarithm of 2, 0.06 is the logarithm of 4, etc.. Giving the film twice as much exposure normally would not result in twice the density on the film. John Schaefer, in his book Basic Techniques of Photography, An Ansel Adams Guide, Book 2, gives a completely lucid instruction on using a step-wedge film for testing AND plotting a curve for Zone System application. John is a fine photographer, PhD chemist, Trustee of the Ansel Adams Trust, and as President of the University of Arizona, founded the Center for Creative Photography in Tucson. A lot of other good info in his two books.