In North America, Avengers Infinity War is running about 12% ahead of Black Panther. This would mean Avengers Infinity War will end at about $760-800 million in North America.
Avengers Age of Ultron had a foreign box office that was double its domestic box office. Avengers Infinity War will open in China next week.
Age of Ultron made $240 million in China, which was over half of its North American box office.
Avengers Infinity War should easily make $760 million in North America and at least $1.5 billion in foreign markets for a $2.26 billion total.
A stronger rewatch performance could provide a shot at Avatar. Avatar made $2.788 billion. Domestic $760.5 and foreign $2.027 billion.