This paper provides a closed formula for the maximum radius of robust stabilizability attainable by a discrete linear finite dimensional controller for a discrete-time plant whose transfer function is subjected to stable infinity-norm-bounded perturbations acting additively on each of its normalized right coprime factors. This is achieved by proving relationships between certain associated discrete-time Lyapunov and Riccati equation solutions. A formula is given for a controller which achieves a suboptimal robustness bound with respect to this type of perturbation.