I also mitre by hand. It's not a difficult mitre. Most important is that the chainstays are exactly the same length, or the back wheel won't sit right. You can only do this if you can fixture it right. I attach the CS before the front triangle as it makes it easier with fixturing.
I weld the ST to the BB first, then add the CS after that, before the front triangle.
I have a bit of square tube with two nuts welded onto it about 80mm apart. This is clamped to the edge of my table. A piece of threaded rod goes through the two nuts, is locked in place with two more, and two more pairs of nuts (locked against each other) define the rear spacing (usually 130mm or 135mm). The dropouts are then put in place and held down with two more nuts.
Knowing the height of the rear axle off the table I use math to work out how far away the BB shell should be and what angle the ST should be at. Then I clamp a piece of angle iron across the table for the BB to rest against. I check everything for level and square with a laser level box. Then check it again.
This gives a pretty accurate jig that doesn't move around so I can now mitre the CS (by hand, using templates). They are tacked at the dropouts first and then at the BB shell. Then with a couple of tacks on it I take it all out and put a true rear wheel in to check, trying it both ways round just in case.
Then I weld it all out. Things will move around a bit but that can easily be corrected. This way things are fundamentally the right length and will be back in the right place once bent back.