On an interior wall, possibly already probed with a stud finder, I guess I'd probably grab a drill. Having removed the shoe molding and being careful not to damage the floor, I would drill two, I don't know, 1" holes where the baseboard meets the floor. These holes would be spaced maybe 3 1/2 apart - and they would represent the base of my arch. I would then probably grab my compass (remember from geometry). After finding the center point between the outside of edges of both holes, and holding the fixed point off of the floor slightly, I would scribe the arch in a dark pencil line. [ . . . ]