I just realized I omitted that I'd start with a provisional cast on for the sweater and then could pick up the stitches and work down to make the bottom. Working from the bottom up and decreasing probably makes more sense, I might do it that way instead. I want to know how you decide to do it. Is this to replace the need for a blanket? My kids would have had something like that all bunched up with their legs exposed unless it were closed at the bottom which wouldn't have worked because they would have tried to walk in it and ended up falling.
I should never overestimate my basic arithmetic skills.