The geometry thing is easy to setup IF you first take round servo wheels - one at the rudder pivot (hinge line) and one on the servo.
IF -all holes used in both wheels are not in same relative positions, the distance from servo to rudder connecting points will change

On a 40% model , as much as 1/8" offset on one hole will cause a noticeably uneven setup.
You can rig up a test setup to proove this, on carboard using wheels and some pin pivots and two lengths of piano wire to simulate the cables.. .