A data center network often employs a Clos-network structure due to scalability nature. A previous design model analyzed blocking probability to guarantee the quality of Clos network. However, the conditions to guarantee the blocking probability in these models are stringent, which shrinks the switching capacity. This letter proposes a model that relaxes network design constraints to maximize switching capacity while guaranteeing blocking probability in a two-step manner, where the proposed model divides intermediate switches into two groups. Numerical results show that the proposed model achieves a larger switching capacity than a conventional one-step model.