Some advice that may help..
Movies generally drop from week to week. Especially films that debut huge.
The reason for this is the subsequent weekend doesn't have the additional benefits of midnight showings, etc. Also, this is what's called "front loaded" when fans of a movie flock to theaters opening night, etc.
In other words, there's nowhere to go but down. Let's say a film debuts to 100mil, next weekend, just as a ballpark figure, start with a 50% drop. So, weekend #2, it makes 50mil, weekend #3, 25mil, etc..
Now, here's where it gets tricky. If a film is especially good it may not drop 50%, it may drop 30-40. If there's buzz on a film, that may also lessen the drop (see: The Hunger Games) as audience who were on the fence may be inclined to see what the fuss is about. But that's not the only thing you have to consider, competition from other films.
Sometimes, genre films can get crowded -- last year we had Harry Potter, Transformers, Captain America, Thor, etc. This can result in audiences waiting for reviews/buzz, or opting to wait for a DVD release. Some summer films are spectacles, and people tend to see them in theaters for the experience regardless (special effect films, superhero films, etc) so they're less immune. Of course, there's other x-factors at play, if a summer is filled with superhero films, the lone romantic comedy might take in extra business just because people want something different.
If you're interested in finding more, check out places like HSX.com which link to industry buzz and help get predictions in order. Check out sites like
http://www.the-numbers.com/ and
http://www.boxofficemojo.com for how films have performed in the past,so you can get an idea of how similar films dropped from weekend to weekend (keeping in mind, again, other factors like competition, etc)
Stuff to make note of.. theater counts, twitter/facebook buzz, etc.
Hope this helps some.