With respect to your first question, it depends whether to the ledgerboard is going to be fastened to the studs, rim joist, or the foundation wall. Please read the article entitled "Problems with joists for porches and decks -- poorly secured ledgerboard" in the Exterior section of Ask an Expert.
It would require 3.5 cubic feet of concrete to fill a hole which is a 4 feet deep and 1 foot round, assuming that you would want the concrete to project several inches above grade level to avoid wood/soil contact.