Nobody can make suggestions: The offset depends NOT from the kind of vehicle, it depends from the placement of your object in the specific graphic.
Best way i think, copy the template, linked by gauthier, in your photoeditor, place your object for each view exactly on the grey squares and then delete or hide the template.
First try is the hardest, i know
Reason for the offset for my boeing: First plane i made was a little Albatros L58. All later planes i aligned on this first plane. With the greater planes i got a problem: Aligned on the Albatros-Template they were to big, the rear of planes was in the next graphic. So i moved in photoeditor the planes complete with rear in their own view with value x. This value is the offset, you need ingame to show the object on right position. See image, shows the problem with an airship (yes, the graphics are greater than 128x128 for the planes). Btw - for the first plane, the albatros, i used as template from an existing plane, something from japanese simutrans site.
edit: sry its lz129, not lz128