I use Kodak photoguides for all my lens extension calculations since they have a dial system in the back of the books that will provide the exposure correction by you indicating what focal length of lens is being used along with how much extension you are experiencing. No math (calculator, etc.) is required. Both the Kodak Pocket Photoguide and the Kodak Professional Photoguide have this. The latter also has an extensive section on view camera perspective and depth-of-field controls as well as guides on camera movements. These can be found on E-Bay for reasonable prices as I'm sure they're out of print.