The manual lathe I work on has a degree scale for the arm. I have the radiuses in mm and the diameters in mm of two zones I have to cut. What formula can I use to determine the angle which delimits the two zones? For example: Say I have an item 15mm in diameter. Inner zone A is to have a radius of 6.50mm and a diameter of 12mm. Outer zone B is to have a radius of 7.50mm and diameter of 2mm. I cut zone A first covering perhaps 14.5 mm diameter. Next I cut zone B. How can I determine at which angle to stop the arm when I'm cutting zone B so that both zones are the correct diameter?