ASTM E1921 is a testing standard to determine the fracture toughness for ferritic steels in the ductile-to-brittle transition range. If the crack front curvature criterion in the standard is not fulfilled, the result is invalid, and more testing is required. However, previous investigations do not focus on quantifying the effect of curvature on Master Curve reference temperature, T0. The numerical and experimental analyses done in this study indicate that, for increasing crack front curvatures, the effect of curvature on T0 is marginal and the obtained T0 tends to provide a conservative estimate. Future investigations on large experimental data sets with the methods provided in this study would help to quantify the trends more in detail. The results contribute to the development of more cost-efficient testing methods and impact long-term operation of nuclear power plants.