Ackerman on a pull-pull system to me sounds like a method to cover up mistakes. I've seen plenty of rudder pull-pull setups at the contests I've been to where both cables stay taught throughout the range of motion.

Take the time to lay everything out symmetrically, make sure your horns are square and the holes lined up with the pivot points, etc.

I do not see anything advantageous to slack in the control system of an airplane, again except to cover up a mistake that would cause excess tension in the opposite line.