Miki wrote:

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?

Clamp a V-block to the drill press table such that the bottom of the V
is precisely (as close as you need it) aligned with the spindle. This
is easily done with a spud in the chuck.

Clamp a good (enough to satisfy your definition of "exactly") level to
the tube. As you move the tube along, make sure the level bubble is
centered before drilling each hole.

Use as short a drill as possible to prevent wandering.

Ted