How big is it?

I would agree with Elizabeth and say your room is on the large size, so buy accordingly.

Smaller room has bigger possibilities for bigger sound.
Headphones have the biggest possible sound you can get.

Let me add to the question. What if half of the back wall in the room is open to the kitchen or another room? Are they considered seperate rooms or do you combine the footage?

I had to calculate the reflections from the "main" back wall (the kitchen wall at 32 feet) and the partial wall dividing the kitchen/dining from the living room.
So when I used one of the popular methods, the 'short' wall was 'bad'... then I found another method, and got some good results.. both the short wall, and the back wall are at resonance minimizing distances from the speakers.
(and don't ask what method.. I forgot)
I would say, the back wall IS the more important wall in your calculations. As the bass waves are what you really want to control the most, and the full wall is the thing that matters. (unless the short wall is at a killer standing wave for that speaker location)

Personaly I would not sum the rooms. I would worry more about your speaker and seating position in the primary room than buying speakers that will produce a given spl in the adjoining room unless that is an important factor. I know others probably have more knowledge than I regarding acoustics in rooms, hopefully they'll chime in.