Tearing is the only way to be sure you are getting a straight edge and if the seam is not centered, who is going to notice?
I use the folding cutting boards for large pieces of fabric. One just fits on a twin bed, two are as big as a queen size quilt. I shove our beds together and have a nice working surface. If you are fortunate to have floor space that big,lay out the backing to measure and cut or tear.
Carol J.