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I just cooked up this little fixture to help locate a front derailleur braze-on. The radius of the inside surface of the boss is 5mm, so I turned a rod to 10mm. The face of that radius is supposed to be 8.7mm in front of the center line of the seat tube, so I milled a flat on the back of my rod that is 8.7mm from the other side. I turned a little groove and then drilled and taped a hole in line with the groove and orthogonal to the flat. The groove helps you set the distance from the center of the bottom bracket. So that little rod works in concert with a tube block and a couple clamps. I haven't brazed the boss on yet and checked its position, but I'm optimistic...