Illumination design often involves the tailoring of optical surfaces to provide a desired illuminance distribution. Tailoring algorithms often make simplifying assumptions, like assuming a point or Lambertian source, and that results in a difference between the actual output and the desired output. By using the virtual prototyping capability of illumination software, simulations can be used to accurately predict distributions with simplifying assumptions removed. In addition, optimization algorithms can be used to adjust the tailored surface so that the match without simplifying assumptions is improved. One optimization approach uses the (simulated illuminance)/(desired illuminance) ratio to compensate the distribution used by the tailoring algorithm. This type of iterative compensation approach can be effective when the difference between simulated and desired distributions is small, but becomes less effective when the simulated and desired illuminance distributions have different sizes or are shifted with respect to one another. These limitations can be overcome by using the difference in position of the simulated and desired cumulative flux distributions instead of the ratio of the simulated and desired illuminance distributions.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.