An important problem in optical multilayer thin film deposition is the adjusting of target uniformity masks to reach the desired film uniformity distribution over the substrate. Calibration tests, that is, special multilayer optical designs, are a widely used tool to determine deposition rates for deposited materials at different substrate points; these rates are used to identify the trim pattern for the target mask. I show that using calibration tests along with special fit algorithms for measured optical spectra provide the relative deposition rates for deposited materials at different substrate points with precision significantly exceeding the error in layer thicknesses of deposited film structure. The proposed method is an excellent tool to trim target uniformity masks. (C) 2004 Elsevier B.V. All rights reserved.