Fractional residual of measured EW to simulated EW (|EW_auto– EW_sim|/EW_sim) as a function of the continuum slope around the Na I D line for simulated spectra of SNe II (top) and SNe Ia (bottom). EW measurements include simulated line-oſ-sight Na I D lines at six different positions around the central wavelength and for three different simulated narrow EW strengths. The vertical dashed line is our adopted cut to eliminate spectra with too large a continuum slope, i.e. a big underlying broad line profile, eliminating cases with more than 30% residuals (horizontal dashed lines).