The domestic grosses of the previous five Potter flicks:
Sorcerer's Stone: $317,575,550
Chamber of Secrets: $261,988,482
Prisoner of Azkaban: $249,541,069
Goblet of Fire: $290,013,036
Order of the Phoenix: $292,004,738
Based on those, HBP is guaranteed at least $250m, probably closer to 275. No way Trek beats it.