These days sequels to successful, well-liked films that launch or relaunch a franchise usually see a substantial increase in worldwide box office when the sequel is released. That should be especially true for this film given the big push Paramount is giving it abroad to increase foreign box office. $400 million worldwide is a pessimistic prediction and a result Paramount would be at least somewhat disappointed with. $500 million is what they're likely hoping for given the budget level and how the last film performed, and the film has a good chance of reaching that goal.
400m would be disappointing, but 450m-500m would be decent enough especially since Trek 2009 had very strong blu ray sales.