you can assume 40% yield from picnics and 50% yield from butts conservatively.
so a 10 pound butt would yield 5 pounds of PP
and a 10 pound picnic would yield 4 pounds of PP
I would suggest going with 1/3rd per person, however, I always have leftovers.
¼ pound is usually the norm and its hard to make a call because we don’t know any of the other items being served , for instance if you said they are having hot dogs too, then that’s a game changer. But for all intents and purposes well just concentrate on the pork
Back to the Main, to discourage the waste I would go with slider type rolls or potato rolls if the price is right, you will get more waste in bread than pork.
visualize this, someone loads near a half a pond of pork on a kaiser takes a bite and decides not to finish it for one reason or another. You would cry looking into the trash can after an event with 75+ people.
Based on a 1/4 pound rule your original 40 pound butts will cook down to 20 lbs which equals (80) - 1/4lb sandwiches.
Pork freezes well, don't worry about making too much, its far worse than to not have enough.
You want leftovers, there is nothing worse than running out of food, especially the main item, worst case scenario, you have pork in the freezer, as said before it freezes well.
Throw a picnic or two in there also.
I have had my best Pulled pork using both cuts together.