Once you determine the bellows extension difference between focusing on the nearest point and the farthest point in the scene you can use published tables to determine the necessary aperture without buying a depth of field calculator. Linhof publishes such tables and others were published in Photo Techniques magazine a while back (I believe these tables are cited in Tuan's focusing article in this home page). I'm sure they've been published elsewhere. The Linhof tables are based on an 8x10 print and if you decide to stop down one stop more than the tables call for with 4x5 (e.g. if you want to be on the safe side or if you think you'll be printing larger than 8x10) they are particularly easy to use because the necessary aperture corresponds exactly to the bellows extension difference. E.g. if the extension is 1.6 mm or close to it, the tables say to use F16; if the extension is 2.2 mm or close to it, the tables say to use F 22, if the extension is 3.2 or close to it the table say to use F 32, and so forth up the aperture scale. So you don't need to carry anything around with you. Of course all of these tables (and, I assume, the Rodenstock depth of field calculator) have to make assumptions about the size of the circle of confusion that is acceptable to you.