|This instructive daguerreotype, found in my great grandfather's memoirs, should help to illustrate what's going on inside the tube.|
You have to be careful though, because you can cut too much off of one of the ends and end up with some of the thinner tubing forming the joint... which may then fold like a coke can. Even worse, tubing often comes with the butts shorter or longer than the spec sheets tell you they should be, so you really have to measure for yourself to be sure.
I held my tubes up to a bike light and stuck the depth measuring tool on a caliper into it, set for the point where the transition was SUPPOSED to start, then made sure that it lined up with the actual change in reflection indicating the start of the slope towards the thinner spot. It can be hard to see what's going on in those things, so I had to bracket the measurement (-10mm is DEFINITELY falling short, +10mm is DEFINITELY going over that line).
I found two discrepancies: my seat tube started getting thinner about 1.5 cm lower than it should have and my chainstays (which are ovalized) are about 2.5 mm too wide and a bit too short. The first won't be an issue provided that I leave the tube as long as possible when shaping the end to fit into the bottom bracket lug. The chainstays will probably need to get just a kiss from the vise to be the right shape. I'm a little nervous about that, but it shouldn't be a big deal.
Now I have everything marked, so I will know EXACTLY when I'm in trouble as I trim stuff to length and shape the ends. Next post will likely be about some more practice welds, which will hopefully be more interesting than all of this measuring.