Correlation between the Gaussian fit’s FWHM and the input v_eq sin i_★ of the synthetic line profiles in the whole 0–50 km s⁻¹ w_eq sin i_★ (0–70 km s⁻¹ FWHM) range. Upper panel: correlation between the FWHM and v_eq (black line) and the relative linear fit (blue dotted line), quadratic fit (orange dotted line), cubic fit (green dashed line), and fourth degree polynomial fit (red dashed line). Lower panel: residuals of the fits. The horizontal grey lines outline the 5% difference between the fit and the data.