Extend it the same amount as the throwing arm extends back. This is basically the "equal & opposite" concept: upper arms parallel and equal bends at the elbows (although the arms can be bent in different directions like one arm up and one arm down a'la Roger Clemens).
Good catch by CoachPaul. I don't consider your shoulder alignment an issue if you get to "equal & opposite". But your stride direction coupled with your starting position on the rubber can lead to a posture shift to the glove side as your shoulders rotate and attempt to square up to the plate. I'd suggest starting on the glove side of the rubber to minimize the posture shift.