This paper proposes and implements a constraint loss correction as a supplement to the ASTM correction factor for the master curve method (ASTM-E1921) for Eurofer97 reduced activation steel in order to enhance the transferability of results between compact tension (CT) specimen sizes. To facilitate the acquisition and scalability of reliable fracture toughness data from one specimen size to another and to apply the Master-Curve method with confidence when using small specimens, the difference in the structure of the crack tip stress field between large and small specimens was modeled. The stress intensity factors and critical volumes/stresses of two specimen sizes (1 T and 0.18 T) were calculated with 3D finite element simulations run over a large displacement range and at a temperature range of -120 to -80 °C and the results were compared with real data from a database of toughness tests of Eurofer97 steel. The K values from a miniaturized specimen without or with ASTM correction and with the proposed combined correction with constraint loss factor (which is variable over the entire loading range) were compared with simulated CT-1 T stress intensity values. It was found that this new correction factor was accurate up to a target toughness of 200 MPa.m^1/2, which represents a major improvement on the current state of the art.