Well Gee, I guess it is a dead link. Sometimes that happens. Maybe bdfelice will find a new host. He has an eight page explanation. I will summarize.

Ackerman is rotary to linear differential. Prboz you are correct if everything is at 90 degrees the movement will be the same. So if the control horn is exactly over the hinge line and the control surface is square, not beveled, and the servo arm is at 90 degrees you are correct. If the horns are slightly behind the hinge line or your surface is beveled you will get positive ackerman. One of the cables will go slack. What you don't want is the horns in front of the hinge line (negative Ackerman). Both cables will go taunt and tear something up.

So the goal is not to have slack in the system. The goal is to have the system NOT tighten as the control surface moves away from neutral. However, having a little slack doesn't hurt anything.

Another way to see Ackerman in action is on a airplane with a single servo controlling the ailerons. Notice that each linkage is attached to a servo wheel forward of 90 about 30 degrees. That induces mechanical aileron differential. The surfaces move different distances.