I fold the top in half vertically and measure on a tape that is taped to a table. If it is 48" X 2 = 96in add extra 6inches so the vertical measurement is 102"
I do the same horizontally. Lets say that the end measurement is 86" I do not measure the backing to the top
After I have found the measurements I will be sure that the backing measures the 102" x 86"
I always measure to an affixed tape. I worked at a quilt store and that is how we would figure out the amount of fabric to buy.