For a 20mm radius and 150 mm long piece of work and a Chester mill Jason's method whilst good and workable won't work in this case.
How accurate does the radius have to be ?
Reason I ask is if you use a fly cutter in a boring head, with the head tilted at an angle you can get a very close approximation to an arc, in fact it will be part of an ellipse.

The more you tilt the head, the smaller the radius, same applies to the flycutter, smaller offset in the boring head, smaller radius. As an indication the rad in the picture was to fit a 1.2 metre long rack to a 3" tube.