Quality of Transmission (QoT) metrics are used to predict and evaluate optical transmission performance, and have received much attention due to recent interest in real-time software defined networking (SDN) control and the challenges posed by open, disaggregated optical systems. In this letter, we address the QoT-estimation (QoT-E) issue, by focusing on real-time operations, where a dynamic control plane improves path performance estimation accuracy by interrogating optical performance monitoring (OPM) devices in the network. We also recognize the difficulty in testing such control plane procedures over large scale systems, which is a necessary step to validate control plane scalability before it is implemented in real systems. Accordingly, we have developed an optical network simulation system that incorporates physical transmission modeling into the Mininet packet-network emulator to enable the development and evaluation of optical control plane processes that depend on optical transmission performance. In this letter, we use this new tool, Mininet-Optical, to evaluate a SDN QoT-E procedure based on the deployment of emulated OPM nodes at periodic locations in an optical transmission system, which are interrogated by a SDN controller in real-time. Through this new capability, we observe behavior that manifests across a large scale system of 70 in line amplifiers and 15 ROADM nodes and are able show improvements in QoT accuracy up to 3 dB.