I think I kind of agree with your numbers too.
300 domestically is a possible number, if not more, Im sure it will get pretty close. But the main question is the foreign market. It could do 400, it could do 500, it could do 600... we just wont know how good it will do until July next year. But I strongly feel the international market will be the surprise number to many of us. In any sense, it could be really huge, or it could be not as good.
But I'd go with the first option, for sure.
This is not a normal x-men sequel, this is Fox's HUGE EVENT movie, so thats the key. An event like this wont work in a similar way to X3, it will do much much more, and lets not forget the 3D release this time.
Id be happy with $800m, that's a huge jump compared to all previous sequels, but if it ends doing more.... oh man... I'll be f**** crazy excited, lol
We'll find out next year
Next Big X-Men Crossover
* 2017- X-Men: Age of Apocalypse
Edit: Forget about 2017:
Official announcement -> X-MEN: APOCALYPSE - MAY 27, 2016!!!!