  Launching a starshade to rendezvous with the Nancy Grace Roman Space
Telescope would provide the first opportunity to directly image the habitable
zones of nearby sunlike stars in the coming decade. A report on the science and
feasibility of such a mission was recently submitted to NASA as a probe study
concept. The driving objective of the concept is to determine whether
Earth-like exoplanets exist in the habitable zones of the nearest sunlike stars
and have biosignature gases in their atmospheres. With the sensitivity provided
by this telescope, it is possible to measure the brightness of zodiacal dust
disks around the nearest sunlike stars and establish how their population
compares to our own. In addition, known gas-giant exoplanets can be targeted to
measure their atmospheric metallicity and thereby determine if the correlation
with planet mass follows the trend observed in the Solar System and hinted at
by exoplanet transit spectroscopy data. In this paper we provide the details of
the calculations used to estimate the sensitivity of Roman with a starshade and
describe the publicly available Python-based source code used to make these
calculations. Given the fixed capability of Roman and the constrained observing
windows inherent for the starshade, we calculate the sensitivity of the
combined observatory to detect these three types of targets and we present an
overall observing strategy that enables us to achieve these objectives.
