Join Date: Feb 2004
Location: St Peters, Missouri
Bikes: Rans Rockst (Retro rocket) Rans Enduro Sport (Retro racket) Catrike 559, Merin Bear Valley (beater bike).
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Life isn't as easy as it used to be.
Back in the days when men were men and bike stems had quills, virtually all stems had a 72 degree angle which roughly matched bike's head tube angles so the stem was roughly parallel to the ground. Stem length was measured from the centerline of the quill anchor bolt to the centerline of the handlebar clamp. Guessing which stem length would fit you was relatively easy.
Stem selection today is a lot more of an art form. Threadless stems today are commonly sold by "extension" which is the length along the stem from the centerline of the fork's steerer tube to the centerline of the handlebar clamp. The problem is that stem angles can be anything and, the more the stem is angled upward, the closer the handlebar is positioned to the rider so the stem affects both height and reach.
My suggestion is to carefully consider where you think your hands to be positioned. Then buy the cheapest stem that will position your hands there. Ride your bike like that for a month or so before spending the money on a nicer stem.