  HST/WFC3's spatial scan monitor automatically reduces and analyzes
time-series data taken in spatial scan mode with the IR grisms. Here we
describe the spatial scan monitor pipeline and present results derived from
eight years of transiting exoplanet data. Our goal is to monitor the quality of
the data and make recommendations to users that will enhance future
observations. We find that a typical observation achieves a white light curve
precision that is $1.07\times$ the photon-limit (which is slightly better than
expectations) and that the pointing drift is relatively stable during times of
normal telescope operations. We note that observations cannot achieve the
optimal precision when the drift along the dispersion direction ($X$ axis)
exceeds 15 mas ($\sim$0.11 pixels). Based on our sample, 77.1% of observations
are ''successful'' ($<15$ mas rms drift), 12.0% are ''marginal'' (15 -- 135
mas), and 10.8% of observations have ''failed'' ($>135$ mas or $>1$ pixel),
meaning they do not achieve the necessary pointing stability to achieve the
optimal spectroscopic precision. In comparing the observed versus calculated
maximum pixel fluence, we find that the J band is a better predictor of fluence
than the H band. Using this information, we derive an updated, empirical
relation for scan rate that also accounts for the J-H color of the host star.
We implement this relation and other improvements in version 1.4 of PandExo and
version 0.5 of ExoCTK. Finally, we make recommendations on how to plan future
observations with increased precision.
