I also re-measured the butts on the tubes, as I'd swapped a couple of them out for new ones. I'm glad I was careful; one of the tubes had the thinned section shifted about 30 mm towards one end.
I'm using some guidelines recommended to me by a talented builder (who coincidentally designed the slant-six lugs that I'm using) about the location of the butts and lugs. He said that I should make sure that the lug tips don't come closer than 20mm from the end of the butt on the top tube and 40mm on the downtube. There's not a ton of agreement about how much of a safety margin is necessary, but I think that it makes good sense for me to err on the side of caution for this first bike.
I marked the tubes at the end of the butt and at the beginning of the "safe area" to avoid confusion.
Mitering is when you cut the "fishmouth" into one tube that lets it rest flush against another tube (the base tube) at a specific angle. Figuring out where to cut can be tricky. Luckily, Nova (where I buy my tubes) has a great little app on their site.
I roughed out each miter with a Dremel, then knocked off the last few mm with a half-round file. That kept me from going too far or heating the paper until it browned, which makes it hard to see what's going on. Notice that the template has two cut-lines on it. One represents the outside of the tube, the other represents the inside. This helps you figure out which way the edge of the tube should be slanted, and how much.
I finished each miter by wrapping the corresponding base tube in sand paper and dragging it through, taking care of any remaining little issues. Then I balanced the base tube on top of the miter and checked the angles of both tubes with my little Wixey Angle Gauge. In one case I was within .1 degree of the correct angle on the first try. With the others I had to do a bit more filing to touch it up.
This weekend I should be able to finally start brazing my front triangle!