Our goal is to specify the retinex model as precisely as possible. The core retinex computation is clearly specified in our recent MATLAB implementation; however, there remain several free parameters which introduce significant variability into the model's predictions. In this paper, we extend previous work on specifying these parameters. In particular, instead of looking for fixed values for the parameters, we establish methods which automatically determine values for them based on the input image. These methods are tested on the McCann-McKee-Taylor asymmetric matching data along with some previously unpublished data that include simultaneous contrast targets.