Distribution of the properties of FRs as a function of wavelength. Blue, green, and red vertical spans depict the three spectral windows of 4MOST (filled) and WEAVE (hatched) high-resolution spectrographs. Upper panel: number of independent relations to derive T_eff from flux ratios per wavelength range. Second panel: precision of individual relations to derive T_eff from flux ratios as a function of wavelength. Third panel: accuracy of individual relations to derive T_eff from flux ratios as a function of wavelength. Fourth panel: flux ratio excursion, ΔFR_{min, max}, as a functionof wavelength. Bottom panel: wavelength spacing Δλ of the two components of a ratio as a function of wavelength.