GSX ALREADY calculate the correct distance to place a marshaller, depending on your eyepoint height, to be sure you would be able to see him.
However, this calculation strategy is used only when the parking is in the open. If there's a jetway ( or you customized the parking to indicate there's a jetway, if the scenery have static ones ), the calculation is made depending on the parking radius, without taking into account the eyepoint height, because it's assumed than where's a jetway, there's usually a terminal nearby and so, using the regular distance calculation would result in marshaller too distant and likely going inside a building.
In this case, you should use the Max Marshaller distance parameter in the customization dialog, and set the highest possible number that will allow you to see the Marshaller, without having him placed inside the building.
It's a scenery dependent parameter, and might be different for every parking, that's why we couldn't supply an universal value. Even so, there's no guarantee that, by setting the marshaller with his shoulders immediately in front of the wall, you could see him from the cockpit of any airplane, it's really depending how the scenery was made.
In real world, the Marshaller would use a staircase or a vehicle, to be in an higher position, or the parking would have a docking system.
We are planning adding a way to raise the marshaller to stand on some kind of platform/vehicle, if the distance is below a certain threshold, with future updates.
