The duplex helical method has higher machining efficiency for face-milling spiral bevel and hypoid gears. An accurate and practical approach for calculating the basic machine-tool settings of spiral bevel and hypoid gears manufactured with the duplex helical method is proposed in the present work. The gear tooth surface vector functions and curvature parameters based on basic machine, and head-cutter settings are calculated. Two types of curvature parameters for pinion tooth surfaces are obtained by utilizing the conjugation of gear and pinion tooth surfaces, and the conjugation of pinion tooth surfaces and pinion head-cutter surfaces at the reference point. Next, the basic machine settings for generating pinions in accordance with the two types of curvature parameters for pinion tooth surfaces are determined. Finally, a numerical examples using the duplex helical method are performed and validated by comparing with experimental results. New hypoid gear software has been developed using the new approach. Keywords: duplex helical method, spiral bevel and hypoid gears, face milling, basic machine settings, curvature parameter Highlights • A new exact calculation approach for basic machine-tool settings is proposed. • Three reference points are used to determine the optimal machine settings. • Based on the optimal approach tooth bearings and functions of transmission error on the both sides are favorable. • Hypoid gear design software has been developed using the new approach.