This article is from the Misc Bicycles FAQs, by various authors.
Finishing the brake bridge off so that it would fit between the rear
seatstays required more than I thought. The bridge length had to be
determined and then the mitering angle had to be determined. Before
this could happen, it was necessary to know how far above the dropouts
this needed to be. The distance and where to measure from would differ
depending on the type of dropouts - vertical and horizontal are
different. For vertical dropouts as I had, the distance was 345mm and
measured from the top of the dropout. The distance between the dropouts
was measured at that point and the angle between the seatstays was
measured at that point. The angle was 16 degrees, which was split to 8
degrees for each side of the bridge. The distance was marked on the
bridge. The bridge was then set into the press again. Again making
sure everything was level and the correct angles set. Both sides were
mitered to 8 degrees at the proper length. The bridge fit perfectly.
Aligning Rear-End, Bending outwards
Before the brake bridge could be brazed in, first the rear triangle was
aligned and spread a little bit. The distance between the rear dropouts
was increased by about 4mm since the brazing tended to bring the
dropouts closer together. The rear triangle was aligned with some of
the same tools as the fork. The dropouts needed to be square to each
other and this was done using the same tool as for the fork. The
dropouts needed to be 130mm + 4mm wide which was measured with calipers.
If they weren't, usually just brute force worked or using a lever
similar to what was used with the front blades. Finally the rear
triangle had to be centered on the center of the bottom bracket again
using the same method as for the fork.