My rule of tumb is measuring the batter with left hand, is from my position I can touch his back legs that mean that I'm far enough. However, I always try to move back just and inch from that measure with fastball because I know I can still frame it without reaching. But from breaking pitches I tell to get may a inch or 2 closer in order to be able to beat the movement of the ball to the spot, so that I could be able to attack the zone and win the pitch