I'm thinking Fred and Larry are wishing you guys would take the garbage bag and tape it tightly around your necks after putting it on over your heads!
Any measurement will have to be averaged as the baro moves all the time unless it is dead calm or pinned wide open with a gale. Open a door? baro changes. Turn on the bath fan? baro changes. As many devices as we have today, I am amazed that a make up air vent is not required in new housing. Boiler rms do in commercial jobs. If I turned on my dryer, by 2 bath vents, and the range hood, along with the central vac, it might suck a window in somewhere. That is leaving out a heating appliance that is sometimes power vented. It all must come in, somewhere. How to achieve it is the question? My take on that would be use your app and find out the average flow, find the average outside temp that is being drawn in, and calculate how much coal is burned to bring that air up to the temp at the baro, as that is the air being sucked out.