Never go past the middle of the catcher’s helmet as you will be exposed to a significantly higher number of foul balls…very dangerous. In fact, moving over behind the catcher is not as preferred as trying to set up higher in your stance so you might be able to see. At advanced levels, the umpire will also make mention to the batter that it’s difficult for us to see if he stands there. By rule, you can’t ask him to move, but in casual conversation many batters will move back and/or the catcher will also move a bit to give you a window. Unfortunately, some times you will just get blocked out and you will have to work very hard to read the catcher receiving the ball to give you enough information to call ball/strikes.