Difference between measured and computed surface brightness in units of σ_RMS as a function of the log $R_{HK}^{\mathrm{\prime }}$ index for the M dwarfs in the sample. Red crosses represent the stars rejected by Salsi et al. (2021), black dots the targets considered by Salsi et al. (2021), and blue stars the new targets added in this work. In this plot, the uncertainty on the surface brightness of each measurement has also been divided by the rms.