What I do is cut 2" squares of the fabrics, then lay them out in various ratios to see what I liked best. If you wanted similar amounts of each fabric, then a single square will do. If you're thinking twice as much dark, then 1 square of each light fabric, 1 square of each medium fabric, 2 squares of each dark fabric, and so on. I lay them on a small piece of batting so that I can hang it from my shelves, then step back and look at it. Then change the proportions and see if you like a different proportion better. Doesn't do much for the actual block layout but you do get a pretty good sense of the color and value proportions.