occurring in a similar regime (i.e. log $>0$). The offset between AC1 and the Zhang et al. formulation has been previously reported by Huang et al. (2012) who suggest that differences in the methods of calculating the stellar masses may account for a systematic deviation of $\sim0.2$ dex. The top panel of Fig. \[fig-zhang-valid\] and explanation by Huang et al. (2012) demonstrate an important caveat for the application of any calibration method – if parameters are not uniformly derived, even ‘perfect’ calibrations will perform poorly. It is therefore vital to apply calibrations to datasets whose parameters have been derived as closely as possible to the original data.

The second plot from the top of Fig. \[fig-zhang-valid\] shows that, in contrast to what was seen in AC1, the  estimated for the GASS sample using Eq. \[eq-zhnag\] has a tendency to be mildly over-estimated. Taken together, the top and middle panels therefore imply a total difference between AC1 and GASS estimations $\sim$ 0.45 dex. As described above, Huang et al. (2012) have suggested that this may be due, at least in part, to differences in stellar mass calibrations. In order to test this suggestion, we can re-derive the best fit linear coefficients 