Auxiliary Tanabe–Sugano diagrams are presented. They enable a rapid and reliable graphical derivation of Dq/B and C/B crystal field parameter ratios from spectroscopic data. Using these values, one can calculate Dq and B by employing the original Tanabe–Sugano diagrams. Subsequently, C can also be calculated irrespective of the value C/B fixed in the Tanabe–Sugano diagram.