When making a value sketch, don’t use the edges of your sketchbook as the boundaries of your composition. Always draw a rectangle to represent the boundaries of the paper or canvas on which you will be painting. This will help you design better corners and edges in your compositions.
It’s important that your sketch rectangle be in proportion to your paper or canvas, or else your composition won’t fit when you try to scale it up to full painting size.
Lay your sketchbook squarely in one corner of the paper or canvas, and sight from corner to opposite corner of the larger rectangle.