We present Herschel PACS observations of the [CII] 158 micron emission line in a sample of 24 intermediate mass (9<logM$_ast$/M$_odot$<10) and low metallicity (0.4< Z/Z$_odot$<1.0) galaxies from the xCOLD GASS survey. Combining them with IRAM CO(1-0) measurements, we establish scaling relations between integrated and molecular region [CII]/CO(1-0) luminosity ratios as a function of integrated galaxy properties. A Bayesian analysis reveals that only two parameters, metallicity and offset from the star formation main sequence, $Delta$MS, are needed to quantify variations in the luminosity ratio; metallicity describes the total dust content available to shield CO from UV radiation, while $Delta$MS describes the strength of this radiation field. We connect the [CII]/CO luminosity ratio to the CO-to-H$_2$ conversion factor and find a multivariate conversion function $alpha_{CO}$, which can be used up to z~2.5. This function depends primarily on metallicity, with a second order dependence on $Delta$MS. We apply this to the full xCOLD GASS and PHIBSS1 surveys and investigate molecular gas scaling relations. We find a flattening of the relation between gas mass fraction and stellar mass at logM$_ast$/M$_odot$<10. While the molecular gas depletion time varies with sSFR, it is mostly independent of mass, indicating that the low L$_{CO}$/SFR ratios long observed in low mass galaxies are entirely due to photodissociation of CO, and not to an enhanced star formation efficiency.
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