Colour map showing the dependence of the O mass on the escape probabilities β_6300, 6363 and β₅₅₇₇, according to Eqs. (6) and (7). Dashed black lines trace the location of some relevant O mass values. The solid green line displays the constant β₅₅₇₇/β_6300, 6363 ratio found through Eq. (6). The position of SN 2024bch in this plane, based on the measurements performed on the spectrum at +154 days, is marked with a green star.