I just measure the width of the back, add an extra 12 - 14 inches to give me 6 - 7 extra inches on each side and then divide that number by 3. That gives me the width measurement I need to cut three long strips from 45" wide fabric. The length matches the quilt top plus an added 12-14".
I then join the three lengths to give me a large enough backing with 2 seams. The seams are off center, look better than a center seam and share the wear.
Sometimes, when deciding on the width of my three backing lengths, I might make the center one a few inches wider than the two side lengths - depends on how much fabric I have and also on the printed design on the backing fabric.
Hope I explained this ok.......