How do you calculate volume of pasta from its weight after cooking down?
Because of the different shapes, different pastas will occupy different volumes of space both before and after cooking.

There are two answers that readily occur to me, one simple one more complex.

The simple solution, of course, is just to make too much.  Cook your pasta, put in the casseroles with sauce or adding the sauce after, depending on your recipe, and if you have too much (you should, you planned for it) let it cool, add mayo, mustard, onion, some pickle relish and make pasta salad.  Or throw in soup.  Or ....

Option two is to measure your pasta by volume when dry, assuming that it approximately doubles in size.  So you'll need to fill one of your casseroles with dry pasta, and that should give you enough to make two casseroles worth, once you have cooked the pasta.