Your horns and linkages MUST be exactly the same. Both control horns must be measured and placed identical to each other. Same distance from the hingeline. Same distance from the CL of the servo output shaft. This means---you gotta glue the tail on straight.

Sorry Rcpilet but the bold part of the statement above is wrong.
As long as the servo is the same distance (vertically) from the stabilizer and the pushrod is at 90 deg to the servo horn and the control horn is identical on each half of the elevator it will work fine.