measuring bag geometry

If you have two bags...
1--- 64" shoulder girth with 9" loft
2--- 67" shoulder girth with 7" loft
...what is the outside girth of the bags? I'm trying to figure what girth bivy I would need.

I think that trying to calculate it will be an exercise in frustration (and probably inaccuracy). I'd suggest pinning a blanket or bed sheet around the bag, with yourself in it, with the desired amount of room, and then measure that.