Selection of the initial boundaries for the Fe II (5169 Å) and Si II (6355 Å) features in a spectrum of SN2020bvc. The ‘observed spectrum’ (grey) is the raw SN spectrum following redshift and emission line corrections. The ‘smoothed spectrum’ (red) is the SG-filtered observed spectrum. The hatched Fe and Si regions denote the user-selected regions in which to perform the spline fit for these features. The final boundaries of the fitting regions were allowed to vary by δ resolution elements with respect to the initial boundaries.