I do 4 oz per sammie an that has always been more en enough on a cheap white bun, folk always comment its alotta meat.
I figure 50% loss on butts, sometimes it ain't that much, but better safe then sorry. So, take the weight a yer precooked butt, cut that in half, devide that pound weight by 4 an it'll tell ya how many sammies that butt gonna give ya. In other words, yer gonna get 4 4oz sammies from each cooked pound a pork butt.
I bought 4 oz scoopers ta besure were servin 4 oz a meat on each sammie.