I have pieced backs with large directional fabrics where I wanted to match the fabric. What I do is to cut one edge straight for starters. Then I would ignore where the seam falls and just cut my backing 8" larger both ways than the top, which would give you 4" on either side. If you are taking to a LA quilter, that could vary based on their requirements. If I don't have a table big enough, I fold my top and backing into 4ths (one fold each direction), match up the folded edges on both the top and backing and cut the backing down from there. I don't worry if it's not exactly 4" or even if it ends up a little more or less on either cut. As long as there is enough extra in case it shifts a little. You can do the same with the batting too, just cut a little smaller than the backing.
You may have some smaller pieces leftover, but you can use them in another project.