Neutron irradiation-induced embrittlement of the reactor pressure vessel (RPV) leads to an increase in the reference temperature (T₀) of the RPV steel and reduces the operating lifetime of nuclear reactors. Fracture mechanics testing of RPV steels before and after neutron irradiation, which reveals the shift in T₀, is often limited by the shortage of irradiated material. To solve this, we tested sub-sized 0.16T C(T) specimens manufactured from already tested SE(B) standard Charpy-sized specimens using the Master Curve concept. The transferability of fracture mechanics data from 0.16T C(T) to standard Charpy-sized specimens forms an integral part of this study. To simplify the testing procedure, based on statistical data, we studied the impact of the slow stable crack growth censoring criterion of the ASTM E1921-21 standard on the determination of T₀. We also present a statistically based strategy for an optimized test temperature selection. We found that the results from the 0.16T C(T) specimens are comparable to the standard Charpy-sized specimens. RPV steels containing higher Cu and P contents exhibit a higher increase in T₀ after irradiation. We also found that the stable crack growth-censoring criterion did not influence T₀ significantly. Our results demonstrate the validity of 0.16T C(T) specimen testing and confirm the role of the impurity elements Cu and P in neutron embrittlement. We anticipate further research linking microstructure to the fracture properties of materials before and after neutron irradiation and the optimization of Master Curve testing using the results from our statistical analysis.