There you go, that is all there is to it. That base is the forward point to which the runner is protected. The base last touched is the rear point of protection. Anything outside that is fair game.
I think we are saying the same thing but it is not coming out right (or I could just be being dense). I'm not sure what the base last touched means...the "point of protection" is whatever base the ump judges they would have reached without the obstruction, the only liability the runner has is if they pass the base the ump judges they would have reached without the obstruction....?right?