> The ruler method is simple and works. <

The problem with the ruler alone is that it requires three hands! One hand to hold a straightedge against the surface, a second hand to hold the ruler and the third to work the TX sticks.

No one has mentioned using a servo driver which frees up one hand as it will hold the deflection as long as needed so that you can measure it. The driver will be useful for every plane you build and many inexpensive ones are available.