Problem with the "calculators" is trying to figure out serving size for a group of people.
For planning, figure 50% loss on the PB and Brisket. So you'll have 10lb PB and 7lb Brisket. If you give a 8oz serving (LARGE) then that means 20 servings of PB and 14 of brisket. 4oz would be 40 servings of PB as you stated and 28 of brisket.
Ribs is a different issue, it's how many bones. BB are small and won't feed much. I serve spares because of that, you'll feed more. Usually 12 to 13 ribs per slab, but on BB that's not much meat, so your talking 72 to 78 ribs basically, that should be enough.
The KEY is how you'll serve them.
Put the sides up front, let them fill their plates.
Serve the PB and Brisket then serve the ribs. If you let them pick themselves since it's a party, they'll probably go for the ribs first.