DV, Kegan's answer/ method is great, but may not be obvious to you. The easiest way to think of it, is to lay out the bow to your fancy. Then start tillering the bow slowly. Once you have the stave cut out to rough dimensions and starting to bend evenly with floor tillering, you put on a long tillering string and begin bending the stave on a tillering tree, or a tillering stick. The most important thing is to never, never, never pull the bow to a heavier weight than your target weight!
If you want a 40 pound bow, just never pull the stave with more than 40 pounds of force. If your lay out is too short or too narrow you will run into difficulties, but most bow building books will help you avoid that.
Badger's no set method takes that advice as given and then works from there to maximize bow speed. Look up his "no set tillering" in other threads. Ron