Well, the magic rule is: extension (distance from lens' rear node to film plane) = focal length * (1 + magnification). So the longest lens you can use is 9" and a bit shorter would probably be better. For most lenses, the rear node is near the diaphragm.
Originally Posted by Bob Carnie
There's another magic rule, to the effect that if the lens is intended to be used at magnifications below 1:1 (that's all taking and enlarging lenses) it should be reversed when used at magnifications > 1:1. The big exceptions are symmetrical (with identical front and rear cells) lenses; this includes Apo-Saphirs (heliar type), some, perhaps not all, dagor types and most if not all dialyte type process lenses.
Good luck, have fun,