Some mills are controlled by PLCs where there is an output that toggles to indicate 1 foot or 1 meter of tube has been produced. This output provides the ideal primary signal to be monitored. In other cases, a proximity sensor could be mounted to detect the keyway on a feed wheel. This would serve as a good alternative as a primary signal. Other signals that are useful to connect to include the cut off saw, in line punches, out of material sensors, and auto/manual indication.
The primary signal would indicate that value adding work is being done, and would be used to calculate linear feet/meters or tube produced, plus feed rate of the material. The cut off saw would indicate how many lengths of tube were made, and in conjunction with the primary signal, how long each piece was.