In recent years, states and districts have responded to federal incentives and pressure to institute major reforms to their teacher evaluation systems. The passage of the Every Student Succeeds Act in 2015 now provides state policymakers with even greater autonomy to redesign existing evaluation systems. Yet, little evidence exists to inform decisions about two key system design features: teacher performance measure weights and performance ratings thresholds. Using data from the Measures of Effective Teaching study, we conduct simulation-based analyses that illustrate the critical role that performance measure weights and ratings thresholds play in determining teachers' summative evaluation ratings and the distribution of teacher proficiency rates. These findings offer insights to policymakers and administrators as they refine and possibly remake teacher evaluation systems.