Since I have to layout the butt placment of the tubes I had to breal out a few books for formulas and such.
For example I wanted to know where to start the butt at the bottom bracket for the downtube. I have to start where the top of the bottom bracket miter is. So I have to figure out how far the bottom bracket miter comes into the downtube. If both the down tube and bottom bracket were both 1-1/2" (38.1mm) in diameter the answer would be easy, 3/4" or 19.05mm. But since the bottom bracket is 1-1/2" and the down tube is 1-1/4", the mitter will be shallower. According to the math I will mark a spot 8.5mm from the end of the tube. That mark will be where the top of the hole saw will go through when I cut the miter.