The point about air return is important. You can't imagine how many houses I've been in where there is no gap under say a bedroom door and no transfer vent in the wall to a hallway, etc. where the return air register lives.
As an example, a typical boot for the average bedroom is 6 x 12. Actual sq in of air flow will be less so let's call it 60 sq in. To equal that, a typical 30" bedroom door would need to be cut 2" above the finish floor to allow free return flow. Don't take my example literally because many times things work well enough w/o having enough room under any given doorway to allow a small cat to crawl under.
Booster fans. And auto dampers. Any large home is going to have a sophisticated duct design. Simply sizing ductwork according to Manual J isn't going to be ideal. Number of bends and a 1000 other factors require 'balancing' after the fact.
Your tech will need one of these called a capture hood. Use it both with the door open and the door closed.
Or you can spend a lot of money guessing.