I try to use up the extra fabric I bought to liven up the back. Some of my backs look better than the fronts. Some are plain fabric, but I try to use what I have in my stash. Size of back depends on the size of the quilt.

Patchwork it!
I have done this many times.

Left over fabric from the front makes great back art on the back. If I have enough left overs I will make extra blocks just for that purpose. If I only have random pieces I will do rail fence strips or just single fabric blocks, whatever it takes to make up the short yardage.

Have you tried the Flynn method where you cut it on the angle and sew it back together using the math to increase the size?

Wow, that is the back? Very nicely done! :)

Here are some photos of the quilt I have been talking about. Excuse the wrinkled quilt top.
The second photo shows the two original fabrics on the left and middle and on the right is the Walmart fabric I got today.

Pat

I was always taught to buy 4 yards. that way there is enough to make a complete dress and jacket or pants and top or even use it for a quilt. Only rarely is it not enough.

I would just piece it unless you really really really have to have it all the same. then you are going to probably need close to 8 yards unless it is queen or king size. I piece mine all the time and when I do I usually do two different fabrics. I like to make sorta racing stripe type thing on the back. it is heck getting it to match up but it looks cool when done. I have done it both ways though. last min comfort quilts usually get the multi pieced with what I have on hand but my DS and DD are getting solid backed with one fabric bed quilts. but then again I the fabric on the back is a large print that looks good as a solid sheet if I decided to put that side up on their bed. honestly though I wouldn't fret about adding another fabric or two as long as you have enough for the length or width it should be just fine.

I have never heard of the rule of thumb? What is it? :)

Being a scrappy quilter my backs are always scrappy...orphan blocks, leftover strips, etc.. The only time I buy something intended for a backing is when the yardage is on sale, then I usually finish the bolt.Sometimes I even use another UFO top to make a reversible quilt! No quilt police live at my house!