I'm building up a bike form an old Schwinn Super Sport and need a new stem. I was going to get a Nitto Technomic stem but I'm not sure how to determine the proper reach.

Is there some metric I can apply to the frame and my body that would get me close to being able to order the right size? I'm going to have to widdle the thing down a little in order to fit it into the 21.1 mm headset on the Scwhinn so it's kind of critical to get the right size.