I always look for a geometric or other form of recognizable shape in the overall shadows of a composition. For example, when I look at the reference source for this (you can do it when painting plein air as well), I see the letter “W” formed by all the dark shadows on the buildings on the sides and in the foreground. The little island median cuts into the bottom just enough to make the shape a “W” as opposed to a “U”. I also see a “T” formed by the sky and main building.
Looking for these shapes when planning your composition will help you to place them on the canvas as well as to create a strong design. The interlocking “T” and “W” above provide a strong structure. Once these basic forms are placed, they also provide landmarks so that elements of the composition and perspective can be seen more easily and therefore placed correctly.