The steps of the zoomlevels are determined in the ZoomLevels line (obviously
) in
SilentHunterIII\data\Menu\cfg\Maps.cfg
But I guess the code that you used to make it into a mapoverlay object also involves some scaling.
Showing a screenshot of the problem might help us. Some might even be able to find the cause in the menu_1024_768.ini code if you show it here.