The extensions beyond the tube intersection centers to the ends of the head tube are not always the same. Not even for lugs from the same supplier throughout their style family. Measuring the tube intersection centers is not that easy. To get data points to match what a CAD program comes up with has always required me to pull off the seat and head angles in relation to the axle centers.
Might make sense to measure a vertical line from the bottom bracket center to the center of the top tube and from there extend to the top tube intersections or offset down a distance below those joints to get a clean dimension.
Last edited by repechage; 01-28-12 at 07:24 PM.