A method is provided for the determination a thickness error in a previously deposited layer using the reflection monitor signal of the currently-depositing layer. This thickness error is then used to compute corrections to the thickness of the currently-depositing layer and the next layer which corrects for the thickness error in the previous layer. The method is stable with respect to noise in the optical monitor signal. The technique is applicable for optical coating designs which are not necessarily quarter wave. The approach avoids the buildup of thickness errors from layer to layer and thus is applicable for very thick designs with many layers. Near the end of a currently depositing layer the monitor signal is used to fit the admittance of the base stack under the current layer. This establishes the parameters in an exact reflectance model used for the thickness monitoring of the current layer.