Again???
Ask the manufacturer of the valve(s) you intend to use. There is no reliable, generic method that I am aware of of predicting torques as the number of variables is huge. In fact I would be getting the manufacturer to size the actuator and take responsibility for the package. Depending on the lengths you wish to go to and the criticality you can get proving tests &c.
Major factors that will affect the torque are (but not limited to):
Size;
Class;
Construction type;
Seat material;
Full/Reduced bore;
Line media;
Line pressure (upstream and downstream);
Temperature;
Frequency of operation / how long sat in one position;
Get the picture? If you have not designed and made the valve you have no chance other than by empirical methods. If you have made the valve you will have done the testing!