I doubled this recipe.
I added 4 cloves garlic and fudged the parmesan because, um, I forgot to buy it, so I dumped in what we had and a bit extra mozzarella, and hoped no one noticed. ;)
I added some dried onion and Italian seasoning to the spaghetti sauce because I bought the cans of store-brand and, well, not so flavorful. ;)
I poured in 1 26 ounce can of sauce, layered shells, poured in a bit more than half of a second can, layered shells, poured the rest of that can and another half, layered shells, and then the rest over the top. It seemed like the right amount to me. I did have cheese mixture left over when no more shells would fit in the crockpot, but all my shells left were broken anyway, so I just dumped them. It wasn't THAT much left over.
I boiled the shells "al dente" and then drained in a colander, rinsed, and put straight back into the pot, but this time filled with COLD water. That's a trick my dad taught me. It keeps the pasta from getting sticky while you're filling it. A great technique for any kind of stuffed pasta (shells, manicotti, etc.)
Jeff cooked it 4 hours on Low then let it sit another 2 or so on Warm. I sent 2 loaves of pre-sliced garlic bread with him. Hopefully they enjoy it but not so much that none comes back for me to taste. ;)