This work presents a systematic test of the performance of a spin-orbit operator founded upon the Wood-Boring-based ab initio model potential method [J. Chem. Phys. 102, 8078 (1995)]. Assuming a separability of the problem into a spin-free correlation treatment and a spin-orbit calculation part, this aim can be reached. We shall show in this publication bath the separability and the high level of quality of the spin-orbit operator applying our method to the even spectrum of Ir+. We shall treat the spin-orbit part by means of the above mentioned spin-orbit operator and cope with the spin-free correlation problem through introducing a spin-free-state-shifting operator, shifting the spin-free energies to empirical values obtained from experiment. The quality of the spin-orbit operator is very high, actually better than estimated in previous calculations which were contaminated by an insufficient treatment of correlation. The procedure established is most efficient : Spin-free-state-shifted spin-orbit CI calculations employing a space of the significant reference configurations plus single excitations lead to very reliable spin-orbit splittings provided that the spin-free states are calculated at a high level of quality.