Example of a double Gaussian fit (purple) for the host galaxy sodium doublet within a spectrum of SN 2OO2bo (black). The wavelength separation between both Gaussians is kept fixed. In this case, the fitted velocity is −37.1 ± 1.5 km s⁻¹ compared to −48.9 ± 16.7 km s⁻¹ (without the uncertainty floor of 35 km s⁻¹) obtained with the flux-weighted wavelength average. The vertical dashed line is the mean rest-frame centre of both lines, and the horizontal dashed red line marks the continuum.