[C II] luminosity as a function of M_mol for galaxies NGC 3627, NGC 4321, and NGC 6946. Each point covers the half beam distance (0.50 kpc for NGC 3627, 0.67 kpc for NGC 4321, and 0.32 kpc for NGC 6946). Triangles represent points of upper limits, where [C II] S/N < 3, while circles represent actual measurements for points where [C II] S/N ≥ 3, with error bars representing ±1σ uncertainty. The solid black line shows the fit from our dataset, and the red filled area marks the 3-σ error, while the dashed line marks the fit from Eq. (8) (Zanella et al. 2018), along with their standard deviation shaded in grey. Large crosses in the upper-right corner of the plot represent integrated values for L_[C II] and M_mol, where [C II] S/N ≥ 3, colour coded by galaxy.