Relative intensification of the Fe II 614.77 and 614.92 nm line pair as a function of magnetic field strength (solid line with the underlying grey curve illustrating ±1σ uncertainty arising from the oscillator strength of the Fe I 614.8 nm blend). The horizontal dash-dotted line corresponds to the observed intensification value with the ±1σ uncertainty indicated by the green rectangle. The long- and short-dashed lines show theoretical intensification curves for different choices of Fe II oscillator strength (K13: Kurucz 2013, RU98: Raassen & Uylings 1998) and blending. The two open circles indicate the line parameter-field strength combinations adopted for the synthetic calculations shown in Fig. 7.