I typicaly have the foundation wall modeled with the footing attached as a sweep, then I create a new floor plan with the view range set to cut through my foundation. So I usually have a level called t.o. footing and that is the level I create my foundation plan from. Note that you can define a wall as a foundation wall, which means it will draw down from the level you draw it at, so you can create the foundation wall type, then from your first floor plan just trace your plan or use the pick tool to pick the path of the wall based on your first floor plan.