From Dave Moulton's excellent bike blog. Be sure to check out his other articles regarding fit:
http://davesbikeblog.squarespace.com...of-fish-h.html

"There is another method for determining stem length which states: “A rider seated with their hands on the drops of the bars, will have the front hub obscured from view by the handlebars.”

This works in the same way; longer body calls for a longer stem and vice-versa. The only thing is that this method could be affected by the head angle of the frame and the length of fork rake. I prefer the length of forearm method because it is simpler. It works for most people but there is a small percentage that it will not. I always say if you are comfortable and happy with your current position, don’t change it. Go by the old “If it ain’t broke” adage."