EWs measured for the Fe I line at 670.75 nm in the synthetic spectra and those derived from the empirical law proposed by Soderblom et al. (1993). Gaia colours have been dereddened using E(BP − RP) = 0.415 A_V, following Curtis et al. (2020). The colour of each symbol indicates the rotation period of the star. The Soderblom et al. (1993) relation was derived from spectra of stars on the left side of the vertical dashed line. The EWs obtained for stars on the right side of the line are therefore obtained by extrapolating that relation to a cooler range of temperatures.