Upper bounds on the EWs of narrow lines with FWHMs comparable to the instrument resolution, 0.023 Å (0.012 Å), for MEG (HEG). We assumed that candidate features are only 2 bins wide. One may derive limits on broad features using these curves until the scale of the feature becomes comparable to that of the instrument calibration uncertainties. Limits on absorption lines are identical when there are 25 counts bin⁻¹ but are systematically larger for ≲25 counts bin⁻¹.