It's going to vary and not matter too much for pros, as they will often lift into
the air and land with the feet how they want them to finish.
like the first Fh here- http://www.youtube.com/watch?v=soADA...feature=relmfu
If you are staying on the ground, it seems you don't want the feet too close to
parallel with the BL and blocking your rotation or putting stress on ankles/knees.
In your vid of Fed warming up, you can see him often twist his foot from 45 off
the net to more direct to net when he is not getting the full lift to change the feet
in the air.
Your first vid is just FYB's version of moving to or stalking the ball and not setting up to