Throttle stick at top, carb wide open. Gives you full throttle.
Throttle stick at bottom, carb almost closed. Gives you the slowest idle speed the motor will do dependably.

(The above is a simplification that really doesn't cover some very important details that're actually simple and easy to do.)

You want to rig your connecting rod from the servo so it's length results in the servo arm being at 90degrees when the throttle stick is centered and so that the carb is halfway open.

And have the connecting rod positioned in the servo arm's holes and the carb's arm holes so that when the throttle stick is at the top, and the carb is wide open, the servo arm is roughly at a 45degree angle. And when the throttle stick is at the bottom, the servo arm is at a 45degree angle the other way.

There is more.........