OK, so a reviewer directed me to the 2011 ASHRAE HVAC Applications Handbook. I have the 2015 Handbook, and there in Section 53.7 is an equation which, adjusted for units, has differential Pa at CFM^2 / (27552 * SF^2). There is a Table 2 which lists leakage area ratios for walls and floors. I will assume a ceiling is like to a floor for this purpose. For walls, only exterior, stairwell and elevator shaft walls are listed in this table, and so I find it odd that this whole thing is being used mainly to determine underpressurization wrt an adjacent inhabited space. So should one consider interior walls not adjacent to stairwells or elevator shafts as like to exterior walls, and should one be using the loose, average, or tight ratios?