Centering the rudder is done by raising (centers) and the adjusting to the tiller in the center position. Tension is adjusted by the knob under the handle assembly. Tension changes by ambient temperature changes, so adjustments may need to be made more often for the best feel.
what I did was I loosened the strings where they wrap around the screws in the steering drum. I then centered the tiller next to the seat and then I centered the rudder at the back of the PA. is that what you mean by "raising (centers)"?
my main question is how tight should the strings be after one tightens the screws in the drums that hold the strings?