thomas28,

If you are buying flies, then a size 12 Adams from Flyshop A will be pretty close to the same as a size 12 Adams from Flyshop B.

The problem lies with hooks that have longer or shorter shanks than "standard" (and i use that term loosely). For example a size 12 Bead Head Prince Nymph is going to be larger (longer) than your size 12 Adams due to the hook type used for most beadhead patterns.

This formula that you speak of sounds intriguing, i have never seen it or heard about it before so i am not much help there. Chances are if it did exist then Dr. Fish would know about it given his position at Mustad.

Hope this helped you out some.