In this extended abstract, we present our initial work on how the reachable sets of the Koopman operator for an iterative method can be approximated and then used to make decisions about the number formats required for implementations. We present our initial framework for learning the Koopman operator and performing reachability analysis on it, followed by an illustrative example on the Gauss-Seidel stationary iterative method, where the reachability analysis can inform the decision to use signed/unsigned variables.