The thickness of carbon overlayers that spontaneously form on the surface of real-world samples needs to be accurately determined to enhance the accuracy of quantitative surface compositional analysis. One method for this determination proposed by Ebel et al. [J. Electron. Spectrosc. 34, 313 (1984)] for use in x-ray photoelectron spectroscopy determines the film thickness based on the area ratio of the C(1s) photoelectron and carbon (KVV) Auger peaks. The Ebel model has been evaluated in this work through the use of self-assembled monolayers (SAMs). The SAM approach permits the comparison of film thicknesses determined by the model with the known thickness of the controlled monolayers. Data from the SAMs consistently fell below the curve predicted by the model. Discussions of potential sources for error are presented with respect to photoelectron attenuation models. Examination of these potential sources, however, could not identify a source of sufficient magnitude to account for the difference between predicted and actual results. It is suggested that the error is due to an incorrect assumption with respect to the normalization factor used in the model. Additional excitation of the C(KVV) due to the large number of energetic photoelectrons originating from the substrate is suggested as the cause for the discrepancy between prediction and experiment. Experimental evidence supporting this assertion, and possible methods of correction are offered.