Your description does not extend all the way to the deflection of the control surface.
If, for the neutral position, the horn of the control surface is perpendicularly in line with the hinge, and the servo horn is perpendicular, the control surface will mimic the deflection of the servo horn in a linear way, regardless the movement of the connecting rod.
Check these related threads:
http://www.rcuniverse.com/forum/m_99...tm.htm#9958827
http://www.rcuniverse.com/forum/m_98...tm.htm#9877110
http://www.rcuniverse.com/forum/fb.asp?m=9227808
http://www.rcuniverse.com/forum/fb.asp?m=10276535
If the geometry of your linkages introduces some differential or non-lineal deflection; then, experimentation would be the more practical approach.
For that, you would need to make two angular scales, one for the stick of the Tx and one for the control surface.