When ion milling is employed to change the contours of large area surfaces it is often assumed that the sputtering rate is directly proportional to the sputtering yield for normal ion incidence on to a plane surface. It is shown how this zero order approximation can lead to errors in expected contours and higher order approximations are analyzed to indicate how errors can be reduced by improved choice of sputtering parameters. The behavior of static, broad beam, and dynamic, swept focused beam systems are discussed.