I would count the number of blocks you need of each color and do the math. I have found that it always takes more that I think when all is said and done. I always buy extra because if I need more it is usually gone. That's how I got a huge stash. I recently made a baby quilt and used all different fabrics. It took me days to figure the yardage--probably 4-5 days on and off. I didn't run out of any of the fabrics. So I guess that it was time well spent.
Sue