Without any element of context regarding the popularity of the Marvel brand, or the popularity of the genre or how well the movie going business is going to be 3 year from now, or how the international markets are going to fare, without any indication of the path this film is going to take, of the line up of characters they are going to use, I can see a slight decrease from Age Of Ultron.
It usually happens in the case of endings divided in two films (see Harry Potter, albeit DH1 was saved by the growth of international markets, Twilight or Hunger Games). Especially if they decide to keep the big 4 (Cap, Iron Man, Thor, Hulk) for IW part 2 and focus on newer characters.
Now I'm not sure wether or not Marvel is going to play "the end of an era" card, especially if RDJ, Evans and Hemsworth get new contracts for Phase 4 and beyond, but it definitely feels like they will right now. A lot can change of course.
As for predicting I would say $1.3B is the floor (especially if Spider-Man is involved), but then again, 3 years from now the chinese market might be just as big or bigger as the domestic one on a regular basis so its really hard to tell.