I made the graphs in Stylecraft Special DK using :-
Black (1002) approx 100 grams
Aster (1003) approx 200 grams
4.00 mm hook
With a double crochet stitch (UK terminology)
The cushion size was a 20 inch square. I did find that my tension changed mid crocheting and if I was making the cushion again I think I would go down a hook size to make it a tighter fit around the cushion. I joined the two parts of the cushion with a round of double crochet in black and then another round in Aster.