I previously drilled the flute tube holes, so all I had left to do was drill the one hole at 90 degrees at one end of each tube.
Having learnt how to draw a straight line along a tube by placing it in a piece of angle aluminum, I built on that to determine a line at 90 degrees to the first.
Placing the tube in the angle again, with the first line flat against one side of the angle, I measured and marked the distance from the corner of the angle to the line. Then I marked the same distance on the other side of the angle from the corner and used that to mark the tube at both ends. Then drew a line between the marks, measured the distance to the hole and drilled it.
Clear as mud? There must be a simpler way, but this is the best I could come up with!