This is used to mark holes for the rings at the top of curtains.

Just mark the center of each hole and then line that mark up with the center of the guide and you can mark the hold that you need to cut for the ring.

I needed the hole to be 43mm but I have included the SolidWorks file and the hole size can be adjusted between 25mm and 45mm as desired (Edit Sketch1 under Boss-Extrude1)