Made the dovetail bar for the new telescope:
At last the cardboard tube is beginning to look like a telescope!
I spotted through the dovetail bar and tube into the lens mount. I then had to figure out how to set up the lens mount so the holes are perpendicular to the surface.
I figured that my rule would follow the surface if placed at the hole position. I could then match that up with the bubble-level. The surface would then be horizontal and thus the hole would be perpendicular to the surface. The method seems sensitive and should be accurate enough.