I am trying to perform a topographic correction of a Landsat 8 image using the C method described in the paper below. I am doing this in ERDAS IMAGINE Spatial Modeler. However, I am not sure how to calculate the value of Ck in the Spatial Modeler. In the paper, the way to calculate it is described on page 1058. It is the quotient of the gradient/intercept of the regression equation, pT vs IL. I have the pT and IL layers but am not sure how to get the Ck value in order to complete the model. I also have ArcMap 10.5.