It is customary to define the thickness of the image layer in rotational panoramic radiography along the central ray of the X-ray beam. This results in calculated values which overestimate its thickness, especially in regions where the central ray deviates markedly from orthogonality. A supplementary definition is proposed which considers layer thickness in a direction perpendicular to the central plane of the image layer. This provides values which may be more easily related to the thickness of the object being radiographed. A mathematical approximation for calculations involving the supplementary definition is described and illustrated with numerical examples.