I tried searching, but couldn't seem to come up with the correct search
term. I'm quite sure this has been discussed though.

If you have a tube with a hole drilled in it radially, and want to
drill more holes exactly in-line with the existing hole, how would you
do this on a drill press? Finding the center of the tube is the easy
part, but how do you know you have the tube "clocked" correctly on the
V-block?