Herein, we present two geometrical models based on an effective point-charge approach to provide a full description of the lowest sublevels in lanthanoid single ion magnets (SIMs). The first one, named as the Radial Effective Charge (REC) model, evaluates the crystal field effect of spherical ligands, e.g. F⁻, Cl⁻ or Br⁻, by placing the effective charge along the Ln–ligand axes. In this case the REC parameters are obtained fitting high-resolution spectroscopic data for lanthanoid halides. The second model, named as the Lone Pair Effective Charge (LPEC) model, has been developed in order to provide a realistic description of systems in which the lone pairs are not pointing directly towards the magnetic ion. A relevant example of this kind is provided by the bis(phthalocyaninato)lanthanoids [Ln(Pc)₂]⁻. We show that a fit of the magnetic properties of the [Ln(Pc)₂]⁻ (Ln = Tb, Dy, Ho, Er, Tm and Yb) allows us to extract the LPEC parameters for the lanthanoid complexes coordinated to sp²-nitrogens. Finally, we show that these effective corrections may be extrapolated to a large variety of lanthanoid and actinoid compounds, having either extended or molecular structures.