I generally use the Spocalc program to find hub and rim measurements, an d calculate using the Dan Halem calculator.
I've found it helps to confirm the hub measurements by measuring. With v intage stuff, you will find hubs not in the database, and when the measu rements are correct you learn how to measure. Measurement I find to be o ff most often is rear center to flange. Older Campy hubs often seem to h ave had longer axles put in, and spacers added.
I know when I built rears for 126mm spacing, I would space it to get the Ultra6 as close to the dropout as possible, and to get as symetric a br acing angle as possible.
Steven Johnson, Kandahar, Afghanistan