The biggest problem I have with cotton/poly is that it doesn't stretch or ease in well when two pieces are a little off. The quilt I'm working on that has c/p sashing was sewn by two people and our 1/4" seam allowances are a little different. I can ease the cotton areas, but not the c/p. That's why I didn't recommend buying more for your quilt. Use as much cotton as you can while mixing in your gingham.