Often you have to sew two widths of fabric together to cover the back of a quilt PLUS the extra at the top, bottom and sides. The amount you leave as an overhang depends on how you will be quilting it. Check wth your long arm quilter to see what they prefer. Mine likes about 5 inches extra on all sides.
Below is my diagram to help explain how I do it.
To calculate the amount fo fabric I need, I use 40 inches as my Width of Fabric. With good quality quilters cotton you sometimes find it can be 42 inches. But I calculate using 40inches.
In the diagram below you see my fabrics that will equal 80 inches wide and the length of the quilt plus the extra.
Lay the two pieces good sides facing. I sew the seam on one side with the selvage still on. I sew about 1 1/2 inches away from the selvage. When it's sewn I cut away the extra leaving a 1/4 to 1/2 inch seam allowance.
I am not a fan of a centre seam on a back fabric although there is nothing wrong with it if you want to leave it so.
If not, you can place the panels good sides facing and sew the other selvage side together in the same way. (see diagram below). Cut away the selvage leaving a seam allowance. You are left with a tube shape.
Using a scissor cut along ONE of the folds. (the fold in the fabric when it comes off the bolt)
When you open up the fabric you will have your seam lines at about the first quarter and the third quarter of the back.
I find doing it this way is less cumbersome and I do have control of the many metres of fabric as I sew.
I like to iron my back seam open so it is less bulky. (another good tip!)