I think Geof has outlined a decent method.
Rather than using a ballnose tool, I'd be tempted to use a tool with the proper diameter to machine the radius of the screw flute. To do this, you'd offset the tool on the Y axis so that you machined along the side of the part with the OD of the cutter. You can then control the depth of cut for succeeding passes with simple changes in Y. You could also step down in Z if your tool or setup lacked rigidity to do full chordal depth of cut.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)