Yeah a ruler is really easy. Just hold it and move the control surface back and forth with the Radio. Look at the endpoints of the surface (looking down for the rudder, looking from the side with elev and ailerons). Make sure the ruler edge is in line with these endpoints and hold the ruler fixed while you keep moving the control surface. Your measurements couldn't be more accurate with this method.

If you really wanted to be a nerd, use one of those soldering jigs (helping hands) from Radio Shack and clamp the ruler in it. Position it so that the ruler lines up with the endpoints and measure away.

Joe