  Protostars are born in magnetized environments. As a consequence, the
formation of protostellar disks can be suppressed by the magnetic field
efficiently removing angular momentum of the infalling material. Non-ideal MHD
effects are proposed to as one way to allow protostellar disks to form. Thus,
it is important to understand their contributions in observations of
protostellar systems. We derive an analytical equation to estimate the
ambipolar diffusivity coefficient at the edge of the protostellar disk in the
Class 0/I protostar, HOPS-370, for the first time, under the assumption that
the disk radius is set by ambipolar diffusion. Using previous results of the
protostellar mass, disk mass, disk radius, density and temperature profiles and
magnetic field strength, we estimate the ambipolar diffusivity coefficient to
be $1.7^{+1.5}_{-1.4}\times10^{19}\,\mathrm{cm^{2}\,s^{-1}}$. We quantify the
contribution of ambipolar diffusion by estimating its dimensionless
Els\"{a}sser number to be $\sim1.7^{+1.0}_{-1.0}$, indicating its dynamical
importance in this region. We compare to chemical calculations of the ambipolar
diffusivity coefficient using the Non-Ideal magnetohydrodynamics Coefficients
and Ionisation Library (NICIL), which is consistent with our results. In
addition, we compare our derived ambipolar diffusivity coefficient to the
diffusivity coefficients for Ohmic dissipation and the Hall effect, and find
ambipolar diffusion is dominant in our density regime. These results
demonstrate a new methodology to understand non-ideal MHD effects in
observations of protostellar disks. More detailed modeling of the magnetic
field, envelope and microphysics, along with a larger sample of protostellar
systems is needed to further understand the contributions of non-ideal MHD.
