Pick off play at first

Question
After the ball leaves the pitchers hand and the base runner at first assumes a lead off, what PORTION of first base must be clear for the runner to get back to the bag on a throw down from the catcher.  Our team recently played a game where the first baseman would position herself slightly in front of the bag ( In the direction the runner was attempting to get back to ) and she would lean over.  Either the runner would always have to dive back with a hand or stop short of the bag and stretch her leg to touch the bag and avoid contact.  Is there any interference on this play?

Answer
When F3 doesn't have the ball, the part of first base that the runner chose to go to must be available.  When F3 has the ball none of first base has to be available.

If F3 blocks the part of the base the runner is attempting to get to without the ball, the call is obstruction.