This work applies a recently developed best-estimate data assimilation and model calibration methodology to the three-dimensional reactor thermal-hydraulics simulation and design tool FLICA4. The experimental information used for calibrating FLICA4 parameters stems from the international Organisation for Economic Co-operation and Development/Nuclear Regulatory Commission boiling water reactor full-size fine-mesh bundle tests (BFBT) benchmarks, which were designed by the Nuclear Power Engineering Corporation of Japan for enabling systematic validation of simulation tools using full-scale experimental data. The following specific BFBT experiments have been used in this work for calibrating parameters and boundary conditions for FLICA4: (a) axial void fraction distributions and (b) transversal void fraction distributions. The resulting uncertainties for the predicted parameters and distributions of pressure drops and void fractions are shown to be smaller than the a priori experimental and computed uncertainties, thus demonstrating the successful calibration of a large-scale reactor core thermal-hydraulics code using the BFBT benchmark-grade experiments.