S_LAMOST vs. S_MWO. We used the 114 common stars between Boro Saikia et al. (2018) and the LAMOST DR6 to determine the calibration factors from S_LAMOST to S_MWO. The green dashed line indicates the best fit given by a weighted ordinary least-square bisector Isobe et al. (1990). The inset highlights the low-activity region, demonstrating a detection limit of S_MWO≈0.15 for LAMOST spectra, below which the calibration depends on extrapolation of the fit. The bottom panel shows the residual.