When the slide carb is pushed all the way in by the servo, there's a little needle on the end of the idle screw, inside the throat of the carb, that keeps the carburetor open ever so slightly (about 1mm, or so). The size of the small opening can be controlled by turning the idle screw, on the outside of the carb, in and out.

The servo saver assembly on your SS keeps the slide pushed in all the way at idle with the help of that spring ring you installed between the two halves of the throttle/brake servo horn. You really want to make sure that the slide moves all the way in, and then the servo moves another 5 degrees or so to compress the spring. This gives you a "true" idle. If you rely on the servo position itself to set the idle, it's called "false" idle.

To answer your question specifically:
Lengthen the ball link so that the slide stops moving, plus a little more. Your servo saver will put pressure on the slide, which will give you a consistent idle.