The Duffy's model is reformulated for Sb3+ and Bi3+ in cubic chloro- and bromo-elpasolites with « 0D » perovskite structure. The new equation allows calculating the energy of the first s2 → s1p1 electron transition from crystal structure data. The Stokes shift pertaining to the Sb3+ emission in Cs2BIMIIICl6 double perovskites is rationalized based on Waber-Cromer orbital radii, in the approximation of the symmetrically breathing quasi-molecular (SbCl6)3- octahedron. This constitutes a preliminary structure-property descriptor to be implemented in machine learning algorithms for further computer-assisted design of double halide perovskites doped with Sb3+.