Calculation of Torque depends on a number of factors : 1) friction at the pivoting point. This can be only mentioned by the manufacturers of the valve. 2) material flowing through the pipes gas, liquid, semi solid. 4) Pressure at which the contents in the pipe are flowing. 5) direction of flow of the contents in the pipeline. 6) time required to open or close the valve. 7) flow parameters at the moment of operation. 8) viscosity and surface tension of the fluid in the pipeline. and many many more. According to me you may ask the manufacturer about the details you require by providing him the necessary information. only then they can guide you properly. Else buy a piece and test it your self. Hope you get the perspective.