Thumbnail sketches before painting - Value studies

Before I begin painting I'll often sketch little thumbnails (1-2 inches).  I'm thinking in terms of three values (light, mid-tone, and dark), in terms of foreground, mid-ground and background–and in terms of small, medium and large shapes.  I'll play around with what it would look like if I make the background a darker value or the mid-ground a lighter value or vice-verse.