I have never used a miter template, and don’t really understand how they would help. Maybe that’s my ignorance but it seems they’d just slow you down, for machine mitering (hole saw).
All you need is a scribed layout line to bring the hole saw down on. Blue it first (Sharpie) if you want the scratch to be more visible.
The shape of the miter, and the overall length of the tube, is handled automatically by the hole saw.
You may wish to start with two scratch marks, but the first cut can be almost anywhere, so generally you make the first cut by eye, maybe to a Sharpie mark, and only then do you get out the precision tools to mark where the second cut goes.
I like a precision rule (aka “yard stick” only more precise and made of steel), with a beam trammel to transfer the measurement and scratch a line where the second cut will go. You can use the top or the bottom of the tube, and scratch to mark the deeper side of the miter (e.g. bottom of a DT at HT) or the shallower side (top of a DT at HT), depending on how you like to hold the tube when cutting. CAD will give you the dimension to use in either case, just remember the lengths are different on the top and bottom of the tube except for the specific case of a TT where HTA = STA (parallel frame abgles).
I like these Starrett-style trammel points, expensive to buy new but here’s a used pair on ebay at the mo for $35
Ah these are even cheaper 'cuz free shipping.
You supply your own beam, which can be steel, alu or even wood, though rigidity is good, so consider steel. Mine is alu and it’s rigid enough with some care not to flex it in use. I apply a slight amount of “preload” while setting it on the rule and then match that, same amount of “feel” while transferring it to the tube, to minimize the effect of flexing the beam.
To mark the second cut on a DT, you need transfer the measurement 90° around the circumference. I use a ring that snaps on, held by friction, with sides machined parallel. Part if off on the lathe, then cut it so it’s a “C” instead of an “O”. A bit hard to describe in words but you’ll quickly see how to do it once you have the tube in hand. Not needed when the cuts are in the same plane, e.g. toptubes.
My hole saw miters almost always come out essentially perfect with no hand-filing needed beyond thorough de-burring. I use fine-tooth holesaws, welded bi-metal type, and replace them when they get the slightest bit dull (they’re cheap enough). Get Paragon (PMW) arbors, cheap and good! Buy enough for all your sizes of holesaw so you can leave an arbor in each saw.
Bringing a holesaw down on a scratched layout line is probably precise to a half-mm easily enough, more like 0.2 mm or better with practice, good eyesight and bright shop lighting. Times when you need more precision than that? Just about never.
If your eyesight isn’t the greatest, bifocal safety glasses or the ultimate, an Optivisor with a strong LED work light attached.
They don’t come with the Zebralight attached, I had to drill holes and zip-tie it on. It’s godlike for hitting a layout line or centerpunch, also for getting tiny slivers out of your flesh with tweezers.
