Square inches of washed fabric:
Suppose I get a piece of fabric from the quilt shop and it measures EXACTLY 36 inches long by 42 inches wide.
I wash all my fabrics before cutting. Some people don't, but I KNOW that my finished quilts will need to be washed again at some point if they are used - which I hope they will be.
I've found that most pieces of cotton fabric shrink some when washed. Sometimes lengthwise, sometimes crosswise. I overcast the ends of the fabric before washing to minimize raveling.
If the fabric was off-grain , I could lose some more inches if I cut fabric from selvage to selvage. If the crosswise fabric grain was straight, maybe only a 1/2 inch or so due to raveling.
So, after I might lose some area due to shrinkage, raveling, being off grain, and the selvages, I maybe have 34 x 40 inches of usable fabric.
Some pattern makers seem to be very close in their fabric needs and others seem to allow for "extra."
Bottom line: I don't think I answered your question as to how much fabric you need for that quilt.
Good luck with it.