I determine adequate stem length by selecting a length that provides a small amount of knee to arm clearance when I'm pedaling on a descent with my hands in the hooks, my fingers are in reach of the brake levers and my upper back is nearly horizontal. With the 11-12cm drop that I use, this requires almost no bend at the elbow. Of course, the stem length must be tolerable with the hands on the brake hoods, which is the position I use for most of my climbing. Too much stem length will most often cause stress and fatique at the shoulder joint. If you get into that problem, the stem is too long.
I use the knobby top of the new Campy ultrashift levers while riding aggressively on the flats, with my forearms horizontal.