After a lot of installations, I've found that in most situations the carb linkage attachment point works out very good at 1" out from the throttle shaft. There are numerous ways to obtain a longer throttle arm at the carb. The easiest is to remove the one that comes on the carb and replace it with a nosewheel nylon steering arm from glow aircraft hardware. The wheel collar that fits the nosewheel steering gear is generally sized pretty close to the size of carb throttle shafts. Drill out a little if needed.

If you use a nylon servo arm it's quite easy to add one more hole between the servo output shaft and the first hole in the servo arm. Use Dubro 2-56 ball links for 4-40 rod at both ends of the linkage for the smoothest linkage operation.

All of the above is keyed on having the correct linkage length. If you measure the length at any positions other than when both servo and carb are at the mid point, the linkage length will be wrong. Everytime. Gas carbs are not linear in operation in any event. Full throttle operation will be obtained when the throttle plate is between 75 and 80% open. That's what throttle curves and expo are there for. They smooth out that last 25%.