I always figure a top or backing at about five yards (maybe more, maybe less) - - if I need more, a border isn't a problem to match up.
So, if we are at fat quarters, that would be two and a half yards of light and 2.5 yards of dark ... = five yards, correct?
Lucky you! You figured it out!
The quilt measurements you sited are 68"x78" = 5'8"x 6'6" which equals
4.42 square feet ... hmmm ... I may be doing the math incorrectly ...
let's try this: 5304 square inches ... in 20 fat quarters, you have 7920 square inches ... that should do it!
You were right fat quarters.
did I do that all correctly?