Don’t Overextend Your Pole
Trust me, speaking from experience you will want to have your boom pool as short as possible when working as a sound recordist. The additional length will become a burden faster as you will experience greater pain holding the weight further out from your body.
Ask For Edge Of Frame
Before the camera starts rolling you will want to establish at what point your boom pole enters the frame and ruins the shot. Start by moving your pole into the shot, and ask your camera operator for the edge of frame, move it slowly up and out of the frame until the camera operator tells you where the edge of frame is.
Now you have this height worked out, look beyond your microphone into the background, and look for something you can use like a line or mark on a wall, to gauge where your edge of frame is.