  The phenomenology of modified Newtonian dynamics (MOND) on galaxy scales may
point to more fundamental theories of either modified gravity (MG) or modified
inertia (MI). In this paper, we test the applicability of the global deep-MOND
parameter $Q$ which is predicted to vary at the $10\%$ level between MG and MI
theories. Using mock-observed analytical models of disk galaxies, we
investigate several observational uncertainties, establish a set of quality
requirements for actual galaxies, and derive systematic corrections in the
determination of $Q$. Implementing our quality requirements to the SPARC
database yields $15$ galaxies, which are close enough to the deep-MOND regime
as well as having rotation curves that are sufficiently extended and sampled.
For these galaxies, the average and median values of $Q$ seem to favor MG
theories, albeit both MG and MI predictions are in agreement with the data
within $1.5\sigma$. Improved precision in the determination of $Q$ can be
obtained by measuring extended and finely-sampled rotation curves for a
significant sample of extremely low-surface-brightness galaxies.
