The other thing you need to look at is how big do you want your finished quilt to be? the top of a queen mattress is 60X80. How much do you want to hang over the edge? 5"? 14" (to cover mattresses)? Do you want a pillow tuck? You need to figure this out first.
I would look thru some magazines for something similar for a base line, then go from there. As justflyingin said, buy more than you think! Better to have leftovers than run out and not be able to match the fabric.