The recipe you mention will make about 2 quarts of sauce....I figure 3 cups per 8 1/2 by 11 casserole....and I put a little more than a pound of pasta in a casserole, about 3 cups of chicken....and 2 to 3 or even 4 cups of other "stuff.
I figured that 3 cups of sauce will dress a casserole...but a little more is better..
However the farfalla won't fill the dish as well....a pound of farfalla takes up less space that fusilli because the bows nest and fusilli absorbs more of the sauce....and fusilli gets "gluey" faster.
I would buy another pound of pasta ( pasta is cheap! and leftovers keep!) and see what fills the casseroles.
I would make the recipe using the 1 cup butter 1 cup flour etc....make the casserold plenty saucy....as it will absorb when cooking....and if you have extra sauce....why freeze it or put it with any extra pasta.
I forget how many and what kind of people you are serving....but for a funeral lunch of mostly the elderly....I get 20 servings to a heaping casserole. For a Church committee retreat, which included lunch....21 people ate 1 3/4 casseroles...but 2 guys are an awesome amount!
For a ladies lunch, 20 to 24 per casserole should be good.
Linda C