The reason most folks who have trouble with elevator differential on this type of installation is because they don't realize that a split pushrod which is not centered in the fuse at the servo automatically induces a bit of differential. The manual tells you to use one of the side slots for the elevator servo, reserving the center one for the rudder servo so the pull-pull will be centered. Assuming the elevator pushrod is even on both sides, a quick look at the geometry will show that, with a non-centered servo, one side of the split pushrod is moving farther than the other; instant differential. One can tweak and bend the pushrod to eliminate this, but it is a tedious trial-and-effect exercise. It was easier for me just to make an alternate set-up and glue that puppy in place, so i could center the elevator servo.

What I did is fabricate a bellcrank for the rudder from a DuBro h/d servo horn and two tiny bearings from my stash of heli junk, on a plywood platform. This is centered in the fuselage, driven by an off-set servo. This allows me to use the center slot for my elevator servo; the pushrod is centered and not introducing any differential. There's a photo of this floating around somewhere, in an OLD thread where this was being discussed.