Mike:

Just had the same issue, and spent a couple of nights looking at the geometry, as did I do something wrong.

If both center distances on the rudder arm and the servo arm are equal, its has to stay tight provided you are also on the center of the pivot point.

I also found this comprehensive explanation of the pull / pull and sort of learnt something.

My solution was to make sure center distances were the same as my rudder horn is metric, and the control arm american standard.

http://members.cox.net/bdfelice/