The next item to be textured are the Flaps. Note that when deployed, the flaps are neither horizontal nor vertical. They are somewhere in between with a typical angle around 45 to 60 degrees.
The idea here is to map the texture file in such a way that a symbol drawn on it will appear the same on the surface of the part in a way that doesn't require a stretch to look right.
The same idea applies to control panels: The texture should be layed out in a manner that if someone draws a round gauge, it LOOKS round on the panel in the aircraft.
In this picture, Joe (I really call him Jurgen the Hitler Jugend) is showing the coordinates of the Left Flap. Unfortunately in this case, the flap isn't quite planar. The inside and outside edges don't align with each other. I didn't notice this in building the model, but don't be surprised if I fix it later. At this point, we will just continue since the concept of what we are doing isn't affected.
Remember from the Pythagorean theorem that the hypotenuse (chord of the flap) can be calculated by H^2 = X^2 + Y^2.
The number we need is the ratio of X / H because we are using a top/bottom texture. If we were texturing fore/aft, we would need the ratio Y / H.
Because the angles are not quite the same, we get 0.549 and 0.619 for the two values of Y / H.