Structural integrity of reactor pressure vessel (RPV) is of primary importance to ensure the long-term safety of light water reactors. The pressure vessel made of ferritic steel presents a ductile to brittle transition and experiences neutron embrittlement that needs to be quantified. The so-called Master-Curve (MC) method, standardized in the ASTM-E1921, is commonly used to determine a reference temperature T0, which indexes the median toughness-temperature curve in the transition at 100 MPam^1/2 for one inch-thick specimens, referred as 1T-specimen. Since brittle fracture toughness is strongly specimen size and geometry dependent, provisions are given in the ASTM-E1921 standard to take into account this effect. The standard also imposes restrictions on the testing temperature range where the fracture tests have to be performed. In this study, we carried out a series of fracture tests with subsized compact tension small specimens (0.18T) to determine T0. In a first step, we calculated T0 by following all recommendations of the standard strictly to get a valid T0 value. Then, we took in consideration fracture toughness data obtained out of the recommended testing temperature range and showed that a reliable determination of T0 can be done under these circumstances. Statistics show that this approach is quite reliable if more than 10 data are considered.