As stated, the servo arm should be a perfect 90º to the link, or in your helicopters case, just as pictured. It's all about geometry on these helicopters. If you have a perfect 90 on the arm to the link, then you will have perfectly equal travel up and down. As a servo arm moves, it's pivoting around an axis, but hte other end of the linkage is on something that ONLY travels up and down. So the further the servo arm gets away from center, the less travel will be forwarded to the swash. Hope this makes sense. But it's imperative that you center that arm.