I normally set mine 3/4" from the end of the shaft some folks like it a little farther down the shaft than that so the fletch dont hit their cheek.
I wouldnt go less than 3/4" IMO
__________________
"Minister of Information for the Royal Kingdom of $Tree"...
The common mistake people make when attempting to design something completely Idiot proof is to underestimate the ingenuity of complete Idiots.
5/8" (bottom of the Bitzenberger jig) works OK for me. The closer the vane is to the end of the shaft, the more control you get, but I doubt that it's a big deal.
__________________
Never trust a bow under 40 (inches).