OK... if you don't cross your cables, then do what RCBlimpPro says... straight servo arm with width equal to width at control horn, control horn swept forward so it's directly over the hinge-line.

However, if you cross your cables, your servo arm will need some offset toward the hinge-line. Here's how you figure out how much offset:

Assume:
Control horn is set over the hine line (always important).
X = Total control horn width
Y = Distance from rudder servo hub to the rudder hinge line

Offeset = Sqareroot[X Squared + Y Squared] - Y
Servo arm width = X

Find a servo arm of these dimensions and install it with the offset ("sweep") toward the back of the plane. Whala, no slack.

If you can't find a horn of the exact dimension you figured, then get one with slightly less offset... too much and your cables will tighten at full deflection.

SWB can set you up, but they don't really know exactly how much offset their arms have... they use a combination of offset and a shorter arm to accomplish the same end goal. He has arms designed for certain control horn widths and certain distances from fuse to rudder. He can tell you exactly where to cut the cable exits so you will have no guesswork... personally that's the hardest part.

Nelson hobby can make you any servo arm offset you want, you just have to ask, but can't help you much past that point. Both companies have been very good to me.

Good luck!