measuring tubes for c-c length
I'm building my first frame, using paper miter templates and am having some trouble figuring out where to miter the tubes based on my intended c-c lengths, ie: I want a 670mm downtube (31.7 mm & 73º HT angle). If my HT is 36mm and the BB is 40mm, how do I figure out where to miter the tube.
I can subtract the 18mm of the HT and 20mm of the BB from the total length, for 632mm DT length, but how do I account for the increased distance to the center of the HT caused by the angle of the DT to the HT? Also, since the DT length is based on the center of the DT, how do I figure out the length of either the top or bottom of the DT?