  Atmospheric parameters determined via spectral modelling are unavailable for
many of the known magnetic early B-type stars. We utilized high-resolution
spectra together with NLTE models to measure effective temperatures $T_{\rm
eff}$ and surface gravities $\log{g}$ of stars for which these measurements are
not yet available. We find good agreement between our $T_{\rm eff}$
measurements and previous results obtained both photometrically and
spectroscopically. For $\log{g}$, our results are compatible with previous
spectroscopic measurements; however, surface gravities of stars previously
determined photometrically have been substantially revised. We furthermore find
that $\log{g}$ measurements obtained with HARPSpol are typically about 0.1 dex
lower than those from comparable instruments. Luminosities were determined
using Gaia Data Release 2 parallaxes. We find Gaia parallaxes to be unreliable
for bright stars ($V<6$ mag) and for binaries; in these cases we reverted to
Hipparcos parallaxes. In general we find luminosities systematically lower than
those previously reported. Comparison of $\log{g}$ and $\log{L}$ to available
rotational and magnetic measurements shows no correlation between either
parameter with magnetic data, but a clear slow-down in rotation with both
decreasing $\log{g}$ and increasing $\log{L}$, a result compatible with the
expectation that magnetic braking should lead to rapid magnetic spindown that
accelerates with increasing mass-loss.
