Histograms of the derived ζ₂ values with the BFL20 method in run 5 (upper panel, high-J transitions) and run 6 (lower panel, low-J transitions). The different colours show different assumptions for the excitation temperature of DCO⁺ and H¹³CO⁺ (assumed to be equal). In particular, the reference values used in the main text are shown in red (hence these data are the same presented in Fig. 1), whilst the blue/green curves show a positive/negative variation of ≈20% of that value, respectively (labelled in the top-right corner of each panel). The median values and uncertainties are shown with the vertical dashed lines and shaded areas and with coloured text in each panel.