The Multiplex web site has a servo torque calculator program at:
http://www.multiplexrc.com/calcservo.htm
You can use this program to calculate the torque that is necessary to give the maximum desired control surface deflection at the maximum airspeed. You can use this maximum torque to find the force that the boot tab must supply by dividing the torque by the distance from the control surface hinge line to the expected center of area of the boost tab. Pick a boost tab size and deflection and put it into the multiplex torque calculator program. This will give the torque required to operate the boost tab. Divide the torque by the distance from the center of area of the boost tab to the boost tab hinge line to get the force perpendicular to the boost tab. Find the vector component of that force that is parallel to the control surface and compare it to the required control surface actuating force. Adjust the design of the boost tab and its deflection and iterate the force calculations until the component of the boost tab force perpendicular to the control surface meets the control surface actuating force requirement.

The above design algorythm seems reasonable to me but I have never tried it. It needs experimental verification.