Ok, so I got up and into the white house ...'ve been through the place from one end and level to the other and always find my way back to the Dome room, where the Super Mutant Behemoth lies dead.....
There is no Dome Room in the White House. You are probably in the Capitol Building. The dome room is called the Rotunda.
From the Rotunda go out the entrance with the barricades (as I recall there is a turret on the right of that doorway as you face it from inside the Rotunda. That goes to a large open room. Then there is a corridor to the right, and then an office area to the left. That should get you closer. Then, as I recall you go through about one more room and corridor to get the the entrance area which opens to the mall.