http://www.boxofficemojo.com/news/?id=4206&p=.htm on predictions and meaning of opening weekend. Money quote:
"In terms of a prediction, Sony is going conservative, anticipating a $38-40 million opening for the film. Given the pre-sale information alone, that forecast looks like a bare minimum as an opening closer to
$45-50 million seems like a reasonable opening weekend range. While that would give the film the largest opening for a live action comedy this year, topping
Central Intelligence's $35.5 million opening last month, the size of the budget and the fact this is being looked at as a franchise starter means it's going to want a multiplier similar to the 4.78 average multiplier
Feig's last three films have delivered."
So if it gets around $40 million as an opening weekend, that represents some fraction of projected gross and is okay for the film if it continues to earn. It has to earn out $144 million total. If it tanks after that it would have a hard time doing that. If it hits something like $50 million it would have to really tank not to at least break even.