I've cut wrong on kits before and it's easy to get the right fabric if you buy a kit from your LQS that they put together. The one close to me will usually have a scrap bin where they toss the leftovers from kits. Depending on the size I need, it might be free or cost just a quarter.
I remember one time cutting wrong and could not find all the fabric I needed. But did find one of the fabrics and bought enough to make a large border so it looked intentional. That's my story and I'm sticking to it!