Accurate measurement of flow discharge in open channels is essential for efficient water resources management and optimal operation of irrigation and drainage systems. This study evaluated the hydraulic performance of portable SMBF flumes under free-flow conditions through controlled laboratory experiments and numerical modeling using FLOW-3D with the RNG turbulence model. Four contraction ratios (r = 0.342, 0.464, 0.561, and 0.726) were tested to examine the effect of contraction on discharge prediction accuracy. Model performance was assessed using statistical indicators including Mean Error (ME), Mean Absolute Error (MAE), Root Mean Square Error (RMSE), Nash–Sutcliffe Efficiency (NSE), Agreement Index (AI), and Kling–Gupta Efficiency (KGE). The results demonstrated excellent agreement between numerical and experimental data, particularly at the mild contraction ratio of r = 0.342, where the relative error was approximately 3% and KGE reached 0.93. Increasing the contraction ratio intensified flow separation and turbulence, resulting in higher prediction errors up to 16.5% at r = 0.726. Overall, the FLOW-3D model showed stable and accurate performance under mild contractions (r < 0.5). The contraction ratio of r = 0.342 was identified as optimal, providing a balance between hydraulic stability and measurement precision. It is therefore recommended that mild contraction ratios (0.342–0.464) be used in practical SMBF flume designs.