Left panel: median values of the photospheric activity proxy, S_ph (in ppm), calculated for the 310 Kepler solar-like stars for which rotation periods were measured (García et al. 2014) as a function of the length in days of the applied filter F (red dots). The solid line corresponds to the associated fit from Eq. (A.1). Right panel: median values of the associated errors (in ppm) of the estimated S_ph of the 310 Kepler solar-like stars as a function of the filter N_day (solid line). The red dashed-line corresponds to the difference in S_ph between consecutive values of the filter N_day.