I'm not familiar with Dressel's book, but have marked out planking bands before in some of my own builds. How are you measuring the distances and how are you marking out the band locations? Before doing anything else, I'd triple-check that your measurements are being taken from consistent points on the same bulkhead/frame (something that I've certainly gotten mixed up on before), and using a material that won't stretch. For instance, string might stretch when you pull it tight around the bulkhead shape or to measure it, but a strip of paper or cardstock won't stretch.
JacquesCousteau's post in What Am I Missing was marked as the answer