Before I start cutting tubes, I wanted to make sure I knew exactly where the butts were, so I made this little tool. It's a simple piece of 3/4" SS with a 1/4-20 hole tapped crosswise in one end. I filed a nice radius on the end of a set screw and loc-tited it in the hole. With a travel indicator positioned correctly, it will tell me the wall thickness about 8" in on either end of the tube. This will ensure that I don't accidentally cut too much off one end of a tube. It also serves as a redundant quality control on the tubing manufacture. Second picture shows the tool in use with my butt start and stops marked with a sharpie. I like to label the wall thickness just to help keep thing straight.