This work applies Taylor’s Theory of Critical Distances (TCD) to assess the fatigue strength of defect-afflicted cast steel components made of G21Mn5+N. Based on radiographs of complexly shaped spatial imperfections, the established TCD-framework leads to a sound agreement of numerical and experimental fatigue strength. Moreover, a novel fatigue assessment methodology is presented, which bases on the projected planar defect shape, featuring an engineering-feasible design method without the need of extensive numerical simulations. About one-hundred additional imperfections referring to ASTM E446 class A and C validate the model, showing an average deviation of about five percent in fatigue strength.