Distortion coefficients c_B (top panels) and c_H (bottom panels), calculated according to Eq. (11) in GR, versus their approximations and , calculated with the quasi-universal relations in Eqs. (12) and (13) (top plot in each panel). The corresponding relative deviations from the PCA are given in the bottom plot in each panel. Panels a and c: refer to purely poloidal magnetic fields; panels b and d: refer to purely toroidal magnetic fields. The dashed black line is . The magenta shaded area comprises all data points and the purple and magenta lines represent the upper and lower bounds of Eqs. (12) and (13). The dark blue lines bounding the shaded blue area mark the 90th percentile error region. The EoS are colour-coded, and ordered in the legend, according to the compactness C = M_k/R_c calculated at M_k = 1.4 M_⊙ in GR: red for the highest compactness and blue for the lowest compactness.