  The Macquart relation describes the correlation between the dispersion
measure (DM) of fast radio bursts (FRBs) and the redshift $z$ of their host
galaxies. The scatter of the Macquart relation is sensitive to the distribution
of baryons in the intergalactic medium (IGM) including those ejected from
galactic halos through feedback processes. The width of the distribution in DMs
from the cosmic web (${\rm DM}_{\rm cosmic}$) is parameterized by a fluctuation
parameter $F$, which is related to the cosmic DM variance by $\sigma_{\rm DM}=
F z^{-0.5}$. In this work, we present a new measurement of $F$ using 78 FRBs of
which 21 have been localized to host galaxies. Our analysis simultaneously fits
for the Hubble constant $H_0$ and the DM distribution due to the FRB host
galaxy. We find that the fluctuation parameter is degenerate with these
parameters, most notably $H_0$, and use a uniform prior on $H_0$ to measure
$\log_{10} F > -0.89$ at the $3\sigma$ confidence interval and a new constraint
on the Hubble constant $H_0 = 85.3_{-8.1}^{+9.4} \, {\rm km \, s^{-1} \,
Mpc^{-1}}$. Using a synthetic sample of 100 localized FRBs, the constraint on
the fluctuation parameter is improved by a factor of $\sim 2$. Comparing our
$F$ measurement to simulated predictions from cosmological simulation
(IllustrisTNG), we find agreement between $0.4 < z < 2$. However, at $z < 0.4$,
the simulations underpredict $F$ which we attribute to the rapidly changing
extragalactic DM excess distribution at low redshift.
