Like mentioned above, check out the Ackerman bit, it is very important to NOT have any negative Ackerman in your setup, some positive (where the line goes slightly slack on the non-pulling side as you go off neutral). And, the length of the horns and the length of the servo arms DO NOT have to be the same length. As long as you do not get any binding at the extreme ends of the throw, you can safely have quite a bit of difference. I use pull-pull on both rudder and elevators on most of my planes and always have different lengths due to whatever is required to get the desired amount of throw. Check out Alan's web site for some good info on using pull-pull.
http://homepages.ihug.co.nz/~atong/