When you're not sure if the composition would work on canvas, you can always sketch on your sketchbook. That's the reason I always carry a 11"x14" sketchbook w/ a charcoal pencil. On the sketchbook, you can make small or even major adjustments on the composition. Or you can flip a page and start over. Once you decided and started putting paint on canvas on site, you shouldn't change the composition any more. Or you just can't for many reasons. You can also test value differences on the same sketchbook. But it would be another topic.