Systematic variation of single parameters around the values of the best-fitting disk model, showing the dependencies on total disk gas mass M_gas  [M_⊙]  (l.h.s.), on total dust mass M_dust  [M_⊙]  (middle) and on outer disk radius R_out [AU] (r.h.s.). The full black line shows the total χ, and the dotted, dashed and dashed-dotted lines show its constituents, χ_Phot, χ_Spit and χ_Line, see Eq. (3). The computed line flux ratios with respect to the observations F_line/F_obs (F_line/(3σ) for CO 3 → 2) are shown by the coloured lines as labelled, with tickmarks on the right axis). The grey shaded box marks the parameter-interval where χ is no more than twice its minimum.