When you set up your linkage lengths, set the throttle lever on the carb at it's half open position. Set the throttle servo at neutral with the servo arm referenced across the servo, or 90 degrees to the case, depending on the servo-arm orientation. You want the mid point servo travel to correspond with a 50% position with the servo arm. Now connect the servo end of the linkage at the innermost hole of the servo arm, and the carb end at the outermost end of the throttle lever. That connection should pretty much establish the needed linkage length. Dial min and max travels in with the radio travel adjust and then set up any curves or expo you may want.