I really don't think that your linkage setup is at optimum...You have a short "horn" at the elevon and a long servo arm with the linkage at the outer most hole at the servo to get enough throw. Does your Futaba tx allow you to set the ATV above 100%??? If so, you should set the ATV as high as it will allow and adjust the linkage to get the proper throw. When properly set up you should have the linkage at the elevon as far away from the control surface as possible and the linkage at the servo as close to the servo output shaft as possible. This will get you the maximum mechanical advantage.

Kevin