While Needful is discontinued (4mm needles), I’ve seen translate in sub yarns and even in thicker yarns, but you’ll need some math.
However, if you make it in Paton’s Classic Merino (like gidgetgirl82 did on Ravelry for a Breast Cancer Walk), I’ve added instruction for a Worsted weight such as Patons Classic Merino 5 mm needles (my fav all purpose Merino).