Due to the limited availability of the vast majority of unreported "dark" (failed) reactions, OCC approaches are used to capture the underlying relationships between composition and synthesisability. To overcome the typical challenge arising from the absence of negative data in OCC problem, where classic evaluation metrics lose their validity, we propose a quantitative approach, the MFD method, as a complementary way to evaluate model performance and determine optimal thresholds for effective distinction between positives and negatives in a query dataset.