Software-defined Networking (SDN) can facilitate the deployment of deterministic algorithms with stringent Quality of Service (QoS) requirements that pave the way for commodity Real-time (RT) networks. However, current QoS approaches are conservative and under-provision the network, that becomes a bottleneck when scaling or upgrading infrastructure. In this paper, we argue that the use of fixed priorities for flows – a common practice in RT networks, is the root-cause of this issue. We develop algorithms that use the global view provided by SDN to develop on-demand, variable priority schemes. Evaluation shows that this “correct by construction” approach increases network utilization while still meeting the necessary QoS requirements, thereby improving network robustness under high utilization.