OK, here's a picture.

Rig the servo arm and pushrod like it's shown with the green pushrod and the connection at "a". That's what you want with the TX stick at half-throttle. That way, when the TX throttle stick is moved up or down the pushrod gets pulled or pushed about the most it can be. And it'll be pushed the same amount that it's pulled.

If for some reason the rigging looks like "b" (with the blue pushrod) when the TX stick is at half-throttle, you got problems. If the stick is moved so the servo arm rotates counter-clockwise, it will pull the blue pushrod a very, very short distance. And no matter how the carb arm is rigged, the pushrod end at the carb is barely going to move.

Same relationship happens at the carb arm.

So you want both ends of the pushrod to make a 90degree angle to whatever arm it's connected to.