Linkage geometry is the main factor,....the left halve and right halve halve to be mirror images, not only in deflection, but in the setup. You want to make sure that the L and R severo arms are as parallel as possible using the sub trim feature of your computer radio .....Once you get that as close and possible, sometimes all it takes as little as half a turn on the clevis or ball link to a couple of turns to get each elevator halve perfectly centered. Its also not a bad idea to get a deflection meter, they come in handy when you want the utmost precision........When you are dealing with one servo per control surface there is no reason to use a match box. It can get more technical than i described, but what i mentioned is a good starting point