The present application discloses a method for selecting an optical configuration for a high-precision scatterometric measurement. A geometric parameterization of a grating is determined, wherein the grating comprises a periodic structure. The geometric parameterization is used to generate a representative set of model structures. An eigenvalue method is utilized to compute, for each model structure, a set of solutions which satisfy a Rayleigh condition within the grating. The Raleigh condition within the grating is satisfied when a vertical component of a propagating mode within the grating is zero. Other embodiments, features and aspects are also disclosed herein.