Are you using the book "The Knitter's Handy Book of Top-down Sweaters," by Ann Budd? According to my book, 72 st would be cast on for size 46" at a gauge of 6 st. per inch. It specifically says, Do not join," and then gives the number of sts to cast on in a particular pattern for each size. I posted a sweater that I just finished, that I called "Not Arlene," a few weeks ago.
I love Ann Budd's books because of her charts. They are so easy to customize.