Since my method of putting tubes together is completely different than yours I can't provide much advice. I 1st braze the seat tube to the bottom bracket making sure the angles match the design and it is aligned. Then I assemble all the rest of the main triangle in my fixture. Because it is the same as a full size drawing it assures the tubes stay true to the design. I've worked out a method of spotting and aligning so that after brazing all the joints everything is where it should be. .