What gap is typical for adequate air flow? I need to trim down a few interior doors. The first two are 36" wide for 100sf or so bedrooms with no individual returns. The master has double doors measuring 60" total with a return in the room. There is one large return in the hall near the two bedrooms. I'm thinking 3/8" for the small bedrooms and 1/8" for the master. Please let me know if this works. I want the heat pump to work as efficiently as possible.