Man, this is making a mountain out of a mole hill. This is not rocket science, folks.

The deflection in mm is denoted for that specific model and is measured at the trailing edge as noted above by Rendegade.

Joe also makes a good point because this is just a starting point for any control surface and will, most likely, be changed by the owner to suite his/her needs.

If it says Ailiron 20mm up/ 20mm down, well, measure that at the trailing edge. Who cares what the degrees of throw is at that point. Set it up and fly it. If you don't like it, change it. Then measure it and figure it out if that floats your boat. Arcsin(deflection/chord)? If you have had Trig, then that's alright. If not, then what's an arcsin?

CGr.