My contractor tells me that the there is insufficient return air for my HVAC system. He calculated the total supply grill area and compared that to the return grill area. The house has 7 supply registers that equal 369 sqin and only 2 returns equaling 250 sqin. Is this the correct way to do this? Is there a more scientific way to determine of the system is getting insufficient return air supply? He wants to put in another return duct and 2 more registers but I'm not sure if he is shooting from the hip. The contractor comes highly recommended but this is a small job for him and he often doesn't retun my calls for days or blows me off if I ask a question. Any advice about how to check if he is doing this the right way would be appreciated.