When calculating the torque requirements for rudder and elevator control surfaces can i subtract the area ahead of the hinge line from that behind the hinge line ?

I would assume that the force on this area equals the force on a similar area behind the line.

This torque calculator uses only the area of the actual control surface.
http://www.csd.net/~cgadd/eflight/calcs_servo.htm

I think some of the fudge factor will bepend on the planform of the aerodynamic balance you're using. If the whole uppler portion is exposed such as on a Piper Cub then it acts more like a wing over that area with a center of lift force located up ahead of the higeline at the 25% chord point. Because the force point is out in front of the hinge line the balance horn affects more than just the portion that uses the balance horn. So it's not just an area for area cancelation in that case. Some fancier math or even testing would be needed to figure out just what the results are.

But to be safe you can just use an area for area cancelation and then run the resulting area and deflection angles through one of the servo load calculators. And once you find the load on the servo be sure to pick a servo that is around double that for the rated torque value. Those torque values are STALL values. And you don't want to operate the servo at or close to the stall value. That's when motors burn out, driver transistors pop and battery currect draw requirements climb well above what they should be for happy normal operation.