If the attachment points for the pull-pull lines on the rudder horn are at exactly 90 deg to the hinge line, you don't need to have a perfect parallelogram to maintain proper line tension. Draw it out on a piece of paper and you'll see what I mean. No matter what angle the rudder is at, the lines will stay the same length to the servo arm. If the attachment point on the servo arm is a greater length than the rudder arm it just means that the amount of movement at the rudder is multiplied by the ratio of the two lengths, same as any control horn/servo arm setup. The lines will stay tight on both sides regardless of the ratio (as long as everything is on center).