  Using controlled injection and recovery experiments, we devised an analysis
prescription to assess the quality of dynamical measurements of protoplanetary
disk gas masses based on resolved (CO) spectral line data, given observational
limitations (resolution, sampling, noise), measurement bias, and ambiguities in
the geometry and physical conditions. With sufficient data quality, this
approach performed well for massive disks ($M_{\rm d}/M_\ast=0.1$): we inferred
$M_{\rm d}$ posteriors that recovered the true values with little bias
($\lesssim$ 20%) and uncertainties within a factor of two (2$\sigma$). The gas
surface density profiles for such cases are recovered with remarkable fidelity.
Some experimentation indicates that this approach becomes insensitive when
$M_{\rm d}/M_\ast\lesssim5$%, due primarily to degeneracies in the surface
density profile parameters. Including multiple lines that probe different
vertical layers, along with some improvements in the associated tools, might
push that practical boundary down by another factor of $\sim$two in ideal
scenarios. We also demonstrated this analysis approach using archival ALMA
observations of the MWC 480 disk (\"Oberg et al. 2021): we measured $M_{\rm
d}=0.13^{\: +0.04}_{\: -0.01} \: M_\odot$ (corresponding to $M_{\rm
d}/M_\ast=7\pm1$%) and identified kinematic substructures consistent with
surface density gaps around 65 and 135 au. Overall, this (and similar work)
suggests that these dynamical measurements offer powerful new constraints with
sufficient accuracy and precision to quantify gas masses and surface densities
at the high end of the $M_{\rm d}/M_\ast$ distribution, and therefore can serve
as key benchmarks for detailed thermo-chemical modeling. We address some
prospects for improvements, and discuss various caveats and limitations to
guide future work.
