Starshades are a leading technology to enable the direct detection and spectroscopic characterization of Earth-like exoplanets. Two key aspects to advancing starshade technology are the demonstration of starlight suppression to the level required for flight and validation of optical models at this high level of suppression. These technologies are addressed in current efforts underway at the Princeton Starshade Testbed. We report on results from modeling the performance of the Princeton Starshade Testbed to help achieve the milestone 10-9 suppression. We use our optical model to examine the effects that errors in the occulting mask shape and external environmental factors have on the limiting suppression. We look at deviations from the ideal occulter shape such as over-etching during the lithography process, edge roughness of the mask, and random defects introduced during manufacturing. We also look at the effects of dust and wavefront errors in the open-to-atmosphere testbed. These results are used to set fabrication requirements on the starshade and constraints on the testbed environment. We use detailed measurements of the manufactured occulting mask to converge towards agreement between our modeled performance predictions and the suppression measured in the testbed, thereby building confidence in the validity of the optical models. We conclude with a discussion of the advantages and practicalities of scaling to a larger testbed to further advance the optical aspect of starshade technology.