This is why I said that while the arms don't need to be the same length, they should be close.

In Inewqban's diagram, he is measuring servo movement and not cable movement. When you measure cable movement, the differences are less.

So much less, that I blew it up 5 or 6 times to give you an idea.

In the illustration below, the servo arm is 10 inches long and the control horn is 20 inches long. With a 30° servo rotation, the cables will tighten by 1/32".

While 1/32" might be a cause for concern, when you scale it down to the norm, you're talking about less than .01 inch (Or less than two turns of a clevis) - and even THAT is if the control horn is TWICE the length of the servo arm.

Really not worth worrying about.