Distortion coefficient c_s, calculated according to Eq. (14) in GR, versus its approximation calculated with the quasi-universal relation in Eq. (15) (top plot). The corresponding relative deviation from the PCA is given in the bottom plot. The dashed line is . The magenta shaded area comprises all data points and the purple and magenta lines represent the upper and lower bounds of Eq. (15). 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.