I would imagine that most radii shown on clocks which seems to be your intended use would be more for decorative purposes and more often that not would be turned freehand with a graver than using a form tool so "look " is more important than absolute accuracy.
Even grinding to a printed template that can be stuck to the tool or cut out and used to mark the tool will do, this picture shows a tool in th emaking, with a hole drilled as I mentioned above to form the half round then the printed template has been used to mark th erest of the shape.

The completed tool in use

And completed part

And a bit of big boy 5" dia hand graving
