You have to understand the custom pushback nodes are not used to just specify an ending point, because that information wouldn't tell enough about the path followed. They act as "attractors" for the program to fit a Bezier curve that will align the airplane with the last part of the path. The airplane will not necessarily reach the attractor point, it will just move enough to clear the last point and end up aligned with the last section of the path.
I downloaded the scenery and had a look at it. It's possible to do what you asked, in two different ways, one that uses the AFCAD that comes with the scenery as it is, and another one that requires adding a new node.
See the attached screenshot, the Blue path will use the existing AFCAD node, while the Red path will require to add a new node (the one highlighted in yellow) connected to the place you need to be at the end.
The first solution is easier, and will put you at a slight angle with the airplane release point. The second solution will require editing the AFCAD file, and will result in the pushback truck doing a fairly complex job since the mathematics over a curve so tight is quite tricky, but it can do it, and you'll end up more aligned with the desired point. The secret was to have the line connecting the new node with the desired one to be aligned with the final heading desired.
Of course, since GSX allows to specify two custom paths for each parking, nobody stops you to add both solutions (and the AFCAD changes), and have them presented as 1st and 2nd option in the pushback menu.
NOTE: due to a bug we are working on right now, you MUST select "restart Couatl" from the Couatl menu if you make a change to the Scenery Customization dialog, otherwise the pushback won't work.
