Sandia Optical Fringe Analysis Slope Tool (SOFAST) is a mirror facet characterization system based on fringe reflection technology that has been applied to dish and heliostat mirror facet development at Sandia National Laboratories and development partner sites. The tool provides a detailed map of mirror facet surface normals as compared to design and fitted surfaces. In addition, the surface fitting process provides insights into systematic facet slope characterization, such as focal lengths, tilts, and twist of the facet. In this paper, an analysis of the sensitivities of the facet characterization outputs to variations of SOFAST input parameters is presented. The results of the sensitivity analysis provided the basis for a linear uncertainty analysis, which is also included here. Input parameters included hardware parameters and SOFAST setup variables. Output parameters included the fitted shape parameters (focal lengths and twist) and the residuals (typically called slope error). The study utilized empirical propagation of input parameter errors through facet characterization calculations to the output parameters, based on the measurement of an Advanced Dish Development System (ADDS) structural gore point-focus facet. Thus, this study is limited to the characterization of sensitivities of the SOFAST embodiment intended for dish facet characterization, using an LCD screen as a target panel. With reasonably careful setup, SOFAST is demonstrated to provide facet focal length characterization within 0.5% of actual. Facet twist is accurate within ±0.03 mrad/m. The local slope deviation measurement is accurate within ±0.05 mrad, while the global slope residual is accurate within ±0.005 mrad. All uncertainties are quoted with 95% confidence.