the first braze of the main triangle, indeed the first of the frame proper, is now done, but alas no photos yet exist to document this momentous occasion.
i started by mitering the seat tube, which is a Deda Uno (the rest of the main triangle is Columbus Zona), to the bottom bracket. then i added the secondary miter to allow the down tube to fit into the BB shell. the secondary had to be more exact. apparently the ST can stick into the BB shell a little way with no problems--i let mine stick in about a mm.
i did a few test fits in the jig, modifying the miters little by little. i made a tool to help with trimming the DT miter: i stapled one end of a long strip of abrasive shop roll to one end of a 1.25" wood dowel, positioning it so i could coil it up the dowel. i sprayed some 3M spray-mount adhesive onto the dowel, waiting a couple seconds, and wrapped the shop roll tight around the dowel, stapling at the end to secure it. thus i had an abrasive mitering stick, a little over 31.7 mm diameter. worked damn well on the first try, no shop roll came loose, though it snagged a couple times.
anyway. my Walter BB shell from Bringheli has the ST-DT cast at 60.5°, but i need it to be 59.69°. after brazing the ST in, i slid the DT in there and measured with my handy new vernier protractor (ebay, baby). looked like ti could bend the angle out to 60° or so with one hand, so i think i'll be ok.
oh i did the ST-BB shell in brass. this was initially just to make it easier to do the DT in silver, as the silver temp won't affect the brass already there, but i found an unintended benefit: i was able to lay up a little fillet on the piece of ST that stuck into the BB shell. i'm just a little paranoid about this joint after seeing the Little Fish failure pics. also i couldn't get brass to flow into the shell in one spot. i flipped it over and was able to flow from the inside out, but i was still a little iffy, so the fillet made me feel better about it.