Evaluation of the SIE-profile (cusp): ratio of Eq. (8)divided by the true value for the same configurations of image pairs and the symmetry assumption as in Fig. 10. The result for (A,B) (black solid line) coincides with the one for (B,C) (red dashed line) a). The ratio of Eq. (9)divided by the true value b). The slope of the critical curve at the cusp, Eq. (13), using the ratios obtained in a) and b) c). The grey-shaded area indicates a 10% deviation from the true ratio.