If your quilt top is 80x108, you need for your backing to be several inches bigger in both directions. If you are sending your quilt to a longarmer, you should find out how much bigger that longarmer wants and work to that size. If she wants the backing 4" larger all around (not uncommon), then your backing needs to be 88x116. That is probably too wide for the 2-width method above, which otherwise works great if you're very careful to keep the seams and the slit down one side parallel.
One thing you could consider is buying extra wide fabric for the back, and then you won't have to piece it at all. I often piece my backs from whatever fabrics are left over from the front, and to do that I measure each piece that is available and then either use EQ or draw the possible designs for the back on paper. Then approach it as any other piecing project.