How early should I go to see Harry Potter and the Half-Blood Prince?

I'm going to Metro Pointe 12 in Costa Mesa,CA. And it's always really crowded there. But it's such a great place to watch movies. Though what is the right time to go to see this movie for tomorrow? I'm buying tickets for the 8 pm show tomorrow and I'm leaving my apartment at 6 pm so I can get into the line at 7 pm to get a good seat. Yes, I know I'm going to stand in line for 1 hour ,but it's def. worth the wait.