How do I compare the voltage difference of 2 boundaries with respect to the position along it?

Hello COMSOL users,

Im am trying to replicate the study attached that was performed at a COMSOL conference but i have ran into complications. (The file is attached below)
I don't understand how to get the voltage difference between 2 points on a boundary. These 2 points must be a part of the boundary (but not specifically defined) and the voltage differential I want to get has to lie directly across from it.
So say I'm working the 2D axial symmetrical plane and have a boundary at r .5 and another at .505. These lie directly across from each other and have the same dimension in z. Using an electrostatics module (or electric current) I apply a voltage to my structure and wish to know the difference in voltage in relation to the z component.

Please let me know if you have any ideas on how I should go about solving this. The paper says that I should use extrusion but I am not clear on how. I have read many different threads and COMSOL examples and still don't fully understand how to use it. I tried using the boundary probe but this did not seem helpful.

Note: I believe this paper has some mistakes in the writing.

Regards,
Stephen