No doubt you will get a lot of more or less complicated answers to this question but in order to keep it simple:

If the wire loosen at full deflection you could either move the rudder horns a little bit forward (ahead of the hinge line) or move the attachment points on the servo horn a little bit backwards (towards the rudder).
If the wires are tight when the servo is in neutral but loosen a little under full deflection that is usually not a problem.

/Red B.