Ah..
The diameter of the tube is determined by the counts of the stepper turning the Acme screw. The contact is only to tell the processor " we've made contact". So in my vision the contact must be very consistent but that is all.
In operation the stepper would go to the upper end of the travel, the "stop" would become the "0" point. Then the steps from there to when the beam touches the tube will be the diameter of the tube. Actually the distance to the "V" structure would have to be calculated into the measurement.
I'm sure you will see the "V" structure will hold different tubes at different heights. I believe this can be calculated and accounted for. If it turns out this calculation starts to effect the accuracy a different tube holder can be fashioned.
Thinking more, a stop could be included at the top of the structure. It would be a reference point for the stepper. And if things work out right it would cancel any effect of the contact actuator.
