Umberto, I haven't set or adjusted anything! In the "customize airport positions" I have always just used your default "*" setting for the marshaller and it always worked.
You said you customized the AFCAD in order to have the jetway ending up in a predictable position (I guess that was the reason for doing that) so, you compensate in the AFCAD for what GSX is doing *NOW* automatically, and better, since GSX takes into consideration the preferred ext.
This means you are now using two compensations, so one of them must be removed so, having edited all your AFCADs, you will have to customize in GSX, by setting the stop position and the marshaller position, in order to de-compensate for the correction you did to the AFCAD.
I have always just used your default "*" setting for the marshaller and it always worked. have no idea what your default setting is in meters
There's no default distance. This is calculated automatically depending on the airplane eyepoint.
GSX will try to set the marshaller as far as possible, depending on the plane.
However, after a certain maximum distance, it will start to assume it *MIGHT* be inside a building, so it will instead use a ladder. The asterisk only means use a default maximum distance, but you can change it, if you know that, on a certain parking, this won't be enough to prevent the marshaller being inside a building, but since the addition of the ladder, it will be a very rare circumstance.
I can't measure distance in FSX scenery.
Sure you can. We placed a ruler that moves with the airplane in the editor for precisely that reason: so you can estimate with a glance the maximum distance.
Not that this is really necessary now, since you can place the marshaller precisely. If you do that, the marshaller position will not be automatic anymore.