Fine-structure line excitation analysis on a gas densitytemperature grid of early-M-type RSG models, including photochemical enhancement of the [C II]_λ158 line flux by a factor 10. Densities and temperatures refer to the location of the chromospheric temperature maximum; blue lines indicate iso-density contours. The colored areas delimit the (n_g, T_g) loci where the model-predicted line-area ratios agree with the observed ones within ±1.2σ_rms: [OI]_λ63/[OI]_λ145 (Gray), [OI]_λ63/[CII]_λ158 (cyan), [O I]_λ63/[S I]_λ25 (red), and [O I]_λ63/[Fe II]_λ26 (green). The solutions in the red-filled area are compatible with all four measured flux ratios with a 60% likelihood (model A). If the [O I]_λ63/[S I]_λ25 ratio is omitted, a second solution emerges (model B). The locations of the corresponding least-square solutions are labeled with black dots. The orange abscissa on the top indicates the volume-averaged magnetic field strengths required for virial equilibrium. For details see text.