See the attatched picture, I changed the engine position of number 2 (longitudinal position from -1,81 m to 0 m), number 1 is still the original values from the GSX database.
The engine position is read directly from the the simulator, so it might not always be correct from a visual point of view, since not all planes follow the same standard of where the center of thrust is to be placed. Some planes use the front of the engine, some use the back, some are placed in any position in between, some are totally misplaced.
That's why we added this customization option to the editor in the first place: if planes were all made using a common standard, there wouldn't be any need to customize this, and we could simply placed the cones automatically and they would always looked right.
The location the GSX editor *starts* with, which is the one used if you won't do any customization, is exactly the one as defined in the airplane.cfg, so it shouldn't require too many changes: just place the pointer in the middle of the engine and the cones will appear some distance front/back from that.