Yeah, I just logged in to note using 'raw' instead comment (bad me). The 'guess' part can be changed to something else (such as a small table for limited ranges).
Another thing to note is for renormalizing vectors, quaternions, etc. ... the guessed value is '1' and normally a single N-R step will commonly give you the renormalization multiplier. (edit: I'm pretty sure I have a better method for refining than N-R, post here if you'd like me to try to dig it up: Kahan-Ng refinement).
(edit: many typos)