UMan, Brian25 -

Getting to the basic principle being danced around;

A double butted tube generally has a long butt and a short butt, although a good number of tubes are seen to have the butt the same length on both ends especially in short/small tubesets. A tube should be marked to identify the short butt end (=don't cut me), but some brands are marked for the long end (= do cut me). Important to know the OEM producers
data sheets and read the order page and take good notes.

However I have on several occasions received a shipment with a few odd tubes mixed into say an order of 10 each Columbus downtubes. I try to always check each tube to locate the start of the taper on each end and then draw a line around the outside of the tube with permanent sharpe pen at the right spot. There are fancy tools for that but a decent caliper can be found. I just took took a pair of 3/16 inch SS rods, sharpened a point on one end and then bent the sharp end to 90 degree about 1/2 inch in. This gives two nice feelers. Then use a rubber band around the other end of them and slip in a piece of small rod or tubing about 3/4 inch diameter of so at the end of the rods. It is then easy to slide one point down the inside of the tube and the other goes down the outside. Siding the tool in and out one will see the rods move when they slip from the thicker butt down to the thinner taper. Put a mark there. Repeat the other end and mark it.

If one end's butt is shorter, make your first miter there without cutting the length down... I think that is the gist of it.

/K