Well, again it doesn`t make a difference concerning the viewable size if the image is 765x459 or 1530x918.
Just the quality is different.
My problem is, that when zoomed in a 765x459 is shown in a size that has a bad quality. A 3060x1836 is fine but maybe it`s to large and a waste of webspace. So I just need to know the size for an image with which it is shown 100% in the zoomed version. This will be the efficient size.
(It`s confusing for me, why the 765x459 is shown with the same zoomed size as the 1530x918 - so it´s not depending on stage or original size in this case).