It sounds to me like a straight geometry problem. When the servos are in neutral, the angular location of each arm must be the same. The horns on the elevator must be in the same location relative to each other and the longitudinal axis of the plane. If you look carefully at the Futaba servo arms you will see letters (a, b, c, I don't remember this well) on the arms. I believe ( I do not recall reading this in the Futaba manual so I may be making this up) the letters describe slight differences in the angular orientation of the arm to the center mounting spline of the servo arm to the servo. Work with those letters to make very small angular adjustments in the location of the servo arm. Sorry if this is a bit confusing. Anyone else have comments on the letters on the multible arm Futaba servo arms?