described immediately hereinafter these three different constraint adjustment procedures may be performed jointly in parallel or in series and not all of the three procedures need to be performed a first constraint adjustment step 62 involves determining if some layers of the same material have similar thicknesses e g within 10 to 20 if so these layers are constrained to the same thickness in step 62 then the constrained design is fed to a step 58 u2032 for a re running of the optimization algorithm a second constraint adjustment step 62 u2032 involves determining if any of the layers may be estimated as having a thickness that is an integer multiple of the thickness of another layer e g within 10 to 20 for instance in the exemplary process shown in fig3 5 the layer thicknesses were constrained such that x 1 2x 2 if so these layers are constrained to integer multiples of each other in step 62 u2032 then the constrained design is fed to step 58 u2032 for re optimization a third constraint adjustment step 62 u2033 includes determination of whether a thickness difference between a pair of adjacent layers is similar to a thickness difference between another pair of adjacent layers for example with reference to tables 1 3 if thickness differences x 1 u2212 x 2 and x 4 u2212 x 5 are similar such that x 1 u2212 x 2 u2248 x 4 u2212 x 5 to within 10 to 20 then one or more of the four layers included in these two layer pairs may be constrained such that the thickness differences are equal to one another such.