You must have the pivot points (if establishing positive Ackerman at the servo end) ahead of the pivot point of the servo arm. If establishing it at the control horn on the moveable surface, then the attach points must be aft of the hinge line of the moveable surface.. You want the lines to be barely snug at neutral with the line not being pulled going slightly slack as the surface is moved off neutral. If you get negative Ackerman by doing the opposite, you get in trouble fast as the line become to tight and can cause binding or cause excessive servo current drain.