Why is ther no formula to figure out port volume?

Is port volume truly a requirement independent of port flow? I always thought that port volume was secondary to flow and to always try to achieve the port flow necessary for your HP requirement with the smallest port volume possible to keep velocity up?

Obviously, port volume will rise as material is removed, but if material is removed from the wrong places in the wrong amounts, all you have is a hogged out port that may flow less than the stock port.