i'm by no means an experianced rc guy.
but heres how i did my rudder on my custom little boat.
after i had the servo inplace, i turned it on and ensured the trim on the radio was set to zero.
i measured the distance between the sevo and rudder and fashioned a pushrod out of a small brass dowel.
if you notice, my rudder has a seperate armiture fasened to it. i left the bolt loose until i had the pushrod in place.
with the system on i set my rudder to center and tightened down the bolt.
if its not perfect, the steering trim on your radio can also be use to ajusted the rudder to center.
my radio also has the ability to limit how far the servo will travel in each direction. i imagen thats not the case will all radio's.