This is a quick one, but there'll be more this weekend.
Here's my home made jig. I've got the front triangle (already brazed) clamped in it. I'm using my incredibly intricate chain-stay angle/length tool to set it up for brazing on the chainstays. I use a digital angle gauge while I jostle it around to find the point where the angles match up with my rattlecad diagram.
Two quick points:
1) This works way better with a rigid piece of metal.
2) Referencing off of the head tube instead of the seat tube should yield better results (with a little math). If the seat tube is forward or back a half degree, you'll most likely be able to account for it with the fore-aft position of your saddle. If the relative angle of your head tube and chainstays is off, your bottom bracket height will change as will some of the steering characteristics.
Cheers, have a good week.