Gary, the servo arm does not actually have to be the same width as the rudder horn but doing so simplifies the process greatly. The easiest way to set it up is to have the two the same span. Then the rudder pivot points need to be right on the hinge line. If they are not and are say 1/8" back from the hinge line then you have to have the same 1/8" offset aft. Ackerman is using offset to the correct side of the pivot to either increase or decrease the tension through the travel. I personally run a touch of negative Ackerman so that the cables actually get a little tighter through the travel. I however use a ball bearing supported tiller arm that the cables are attached to. This keeps that additional tension off the servo output shaft. When the tiller or servo arm has the same length as the control horns and you do not cross the cables then your cable exit will be farther forward on the fuse. Sometimes you need to think function over looks. I hope this and the pictures help. Feel free to ask questions not only on this subject but anything else about aitplane setup.