Of course you're nuts. So what?
Originally Posted by John Bartley
You've characterized where all lenses of "normal" (not telephoto, not retrofocus) construction sit when focused at infinity. More-or-less 1.0 * f from the film plane. So what? And why limit yourself to lenses of normal construction? I mean, if you're contemplating a relativley low-budget DIY project then lenses from aerial cameras should appeal to you.
Until you know the lens' focal length and how much closer than infinity you'll want to focus, you won't be able to calculate how much focusing travel you'll need. FWIW, my rule of thumb for lenses to be used at normal distances is that I want the lens to focus to 1:10; this requires focusing travel = 0.1f, max extension = 1.1f. Extension is film plane to lens' rear node, not to the lens board.
Cheers, have fun,