First panel: Linear fits on S_Hα versus S_Ca, for the observed coverage in S_Ca. The colour code corresponds to the number of nights for each star: between 10 and 20 nights (black), between 20 and 50 nights (red), between 50 and 100 nights (orange), and higher than 100 (green). Second panel: Slope (S_{H α} versus S_Ca) versus average S_Hα for quiet stars (S_Ca below 0.17) with T_eff in the 5700-5900 K range (upper panel). Third panel: Same versus metallicity. Fourth panel: Same versus global correlation.