Lines of constant equivalent width in the S_H – A(O) plane, as computed from the grid of 3D non-LTE synthetic line profiles for the three triplet components. The thick (red) solid line refers to the EW measured in the Neckel disk-center spectrum, while the thick (green) dashed line is for the measured EW in the Neckel disk-integrated (flux) spectrum. The solution (S_H, A(O)) given in the legend is defined by the intersection point of the two lines (red diamond). The black diamonds define the uncertainty range of the solution, assuming an EW measurement error of ≈0.5%, as indicated by the thin lines offset from the original S_H – A(O) relations by ± ΔA(O) = 0.0025.