In the paper Rational BRDF [PSCS∗12] Pacanowski et al. introduced a new algorithm to fit BRDF data. The fitting algorithm is an extension of the Rational Vertical Segment Interpolation method introduced by Salazar Celis et al. [SCV07]. In this document we clarify some technical and numerical aspects of the fitting procedure that need to be taken into account in order to get satisfactory fitting results.