Calculating Roof Ventilation for Vaulted Ceilings

I've done a lot of reading on how to calculate proper roof/attic ventilation. All of the calculations start by figuring out the attic floor area required to vent by measuring the attic floor length and width, and multiply to get the area. I've got a 80 year old house with partially vaulted ceilings in two bedrooms upstairs, which means I've got a very small attic floor area above those. My question is, should I be using the full length and width of the roof (i.e. the upstairs floor space) to calculate my ventilation requirements or just my small attic space? I figure that I need the entire floor space. I have open soffits at the base of the roof, and have good space between the roof deck and insulation to get good airflow from the soffits, up the roof deck to the attic. I will be adding ridge-vents across the ridge of the roof, and installing round louvre vents between the rafters at the open soffits. I just need to know the proper attic area to use, so I can figure out how much net free area I need.