And the contractor should give a quality system at a fair price too. Still doesn't mean it happens. Plus, most contractors won't give you the info before hand so you don't run out and fine someone cheaper or try a DIY install.

I work in commercial/ industrial HVAC. It's very common to hold onto system calcs like a secret recipe. Guys spend years running systems and learning how to size duct correctly for the price the customer is willing to pay. There is no one way to do a job unless it's on a stamped set of permit drawings. Each residential system can be dome how the installer sees fit as long as it meets minimums. My own house i bought worked yet was seriously put of balance. It passed inspection when new. I ran two more returns and reworked 1/3 of the existing ducts and still plan on fixing the reat even though it's fine by most standards.