The exact setup depends on how your throttle cable is routed though the firewall and via the bracket on the bottom of the oil pan. The end of the cable can end end up left or right of "optimum" at the throttle connection.
My Avistar servo has a straight throttle arm, so I didn't need any special spacer .. I did use two small washers on either side to prevent the rod end from touching either the big washer or throttle arm when it's rocked back and forth
The older Van's assembly drawings show the servo with an offset arm; the 1/2 inch spacer is there to move the attach point inwards to where the rod end naturally wants to be. I'd use the plans as a suggestion about sizing of the spacer; I could see it varying a quarter inch either way. YMMV.
Here's my setup:
Here is the Van's drawing: