For cislunar applications, the performance requirements for a guidance, navigation, and control (GN&C) system can be derived optimally using a systematic approach. GN&C space systems continue to increase in complexity and support a variety of demanding mission objectives. The multifaceted interactions inherent in a GN&C system make deriving performance requirements a challenging and laborious task. For decades allocating these GN&C specific requirements has been a time-consuming, ad hoc, and iterative process that relies on engineering heuristics, flight experience, and judgment. This thesis demonstrates an alternate approach that determines GN&C performance requirements in a systematic, optimal, and consistent manner. The versatility of this approach is demonstrated by calculating the GN&C performance requirements for two cislunar trajectories, given multiple mission requirements and performance constraints, different optimization criteria, and varying phases in the design process.