Cutting the miter: I use a 3/4" dia serrated roughing endmill to cut the miter. I program the mill to cut a radius .010" bigger than my mating tube. This extra seems just the right amount to limit hand finishing.
The serrated tools are really good because the tool pressure is up instead of in. This means it is less likely to fold a tube. They also last forever. I've never replaced this tool. They are expensive at about $60, but they pay for themselves.



Finding the centerline of the tube with an indicator. Sweep one side, then sweep the other, when they read zero, you're on center.


Here is my setup in the mill. The extra tall vice jaws are only required when doing steep miters like the DT. You can just make out the angle block on the fixed jaw.
This one is easy cause it's only 2 deg.



To set the angle of the cut, I use angle blocks. These go down to 1/4 degree. That seems to be accurate enough.



Top tube clamped with matching blocks. By rocking the blocks on the surface plate, I can tell if they are aligned. With a little trial and error, I can get them near perfect.



I finally got some pictures my tube mitering technique. I use tube blocks made of aluminum. They are matched sets. For most tubes, I only need to use one at a time. When I cut the top tube, I use both so I can keep the miters timed to each other. If I made these again I would make them shorter.