The real question is what other food was available? That alone will make a big difference in how you estimate.
Your math is right, though. 24 butts at around 8 pounds, assuming 50% yield, gives you about 250 6 oz servings. So no problems there.
But if there was a lot of other food out there for people, and a lot of people made sandwiches, I think the most likely scenario was that people took far less than 6 oz of pork. Like you mentioned, probably far closer to the 3 oz range, which is half of what you were estimating.
I know that when I've served pulled pork and put buns out to make sandwiches, people actually take less pork. Especially if you're just using the little white 4" buns. It doesn't take but 3-4 oz to make meal of a sandwich for most people.
So, given you were providing an option for sandwiches, and I'm assuming there were plenty of sides and other food out there, 6 oz was just a high estimate. When I will be serving sandwiches I tend to stick to a 4 oz estimate and there's still usually some left over.