I honestly don't think it has the faintest chance of hitting $300M. Has the total domestic take for any single 3-day weekend ever hit or exceeded $300M? The highest I've seen on BOM is in the upper $200M range, even on weekends with individual films that took in over $200M (TA & JW). The 4-day Memorial Day weekend in 2013 saw $313M in ticket sales, but TFA won't have that Monday boost helping it.
Basically, I'll believe $300M if/when it happens. And maybe not even then.