Excel workbook used to fit rheometer data to the Herschel--Bulkley model and calculate Bingham numbers for non-Newtonian model fluids used in annular displacement experiments. The workbook is designed so that shear-rate data are inserted in column A and shear-stress data in column B. Excel Solver is then used to minimize the fitting error between the measured rheometer data and the Herschel--Bulkley model by adjusting the yield stress, consistency index and flow-behaviour index. The fitted parameters are used to calculate the model shear stress, effective viscosity and Bingham number at the characteristic shear rates corresponding to the low- and high-flow cases in the NORCE annulus. The workbook includes an instruction note showing the required Solver setup. Orange cells contain intermediate calculated outputs, while blue cells contain the final Bingham-number outputs. The workbook was used to calculate Bingham numbers for the selected Block B/C/D fluid pair in the thesis: 0.30 wt% Carbopol with 0.20 wt% NaOH5% as the displaced fluid and 0.60 wt% xanthan with 5.0 wt% sugar as the displacing fluid.