I don't often use wide backing, but I when I do any larger quilt, this is what I do with it:
* I fold the backing in 1/4's (half one way, half the other)
* fold the top in 1/4's
* set the folded top on top of the folded backing matching how they are folded
* cut the backing bigger than the top by however many inches I want it. If I want it 8" bigger overall, I cut the folded backing 4" bigger than the folded top.