just poked in, so i don't know if anyone directly answered.
a backing for a generous queen quilt - 96 x 96 - needs at 3' of 108" or 9' of 40" seamed fabric. add in your amount for seam allowances on the front.
the more pieces you have in the design. the more you need for seam allowances. if you know anyone with a computer program, it can be broken down for you.