A typical size for a twin quilt is 70x90. That's 6300 square inches, or about 4 1/2 yards of 40" wide fabrics, if you were just sewing 40" strips of fabric together with no design at all. So at a minimum you would need about 4 yards of fabric to add to the panel. In reality, you really need to know what pattern you're going to use. Chances are you will want some of the fabrics to be solids or blends, and the proportion of these to the JD fabrics will vary depending on the pattern. And don't forget about binding!