What I usually do is take two lengths of 1/8 ply, about 6 inches long, and clamp them to the top and bottom of the wing so that the aileron (in this case aileron.. same method can be done to the other surfaces) is sandwiched inbetween the two pieces of ply. This is done, of course, with the linkage disconnected. This centers up the ailerons to the surface. If a flat bottom wing, then this becomes even easier by using only one piece of ply on the bottom of the wing and clamping both the wing and aileron to this piece of ply.

Anyway, this centers the aileron so I can attach the linkage. This is my starting point for deflection. After centering the aileron (rudder or elevator as applicable), attach the linkage, make sure it remains centered, then remove the clamps and plywood.

I then use a deflection gauge similar to the one shown in the picture. Then you can see the deflection right from the gauge.

I have also used another method is using a laser for this. A friend sent me a pair of lasers that operate on a 4.8 volt NiCD battery pack. A deflection gauge is made out of an aluminum yard stick and is mounted on a platform behind the surface being measured. The laser is taped to the surface after the surface has been centered, and the narrow laser beam creates a point on the yardstick. Then, you can move the surface up and down to see where the deflection goes on the yard stick. If you want to know the degrees of deflection, well, a little math will do that for you. I made an Excel spread sheet that converts the measurement to degrees. Note that this method shows the correct degrees no matter how far the laser is from the yard stick. The point of centering is the known, and one side and one angle are always going to be the same (that time) 90 degrees at the yard stick and the length from the laser to the yard stick.

Hope this helps and is not to complicated.

DS.