Anton was using 320 feet elevation (32nd floor) and 1300 feet horizontally to the concert (I don’t know what the actual numbers are, I was just using his.) So, approximately 4 feet horizontally for each foot vertically. Same ratio would apply inside the room, so 36 inches back horizontally from the window with about a 9 inch muzzle elevation off the carpet would give a direct line of sight. Any compensation for bullet drop would just reduce that angle.
The triangle is from the muzzle end, down vertical to the ground, then on the x axis out to the impact zone, then the h axis back to the window. If he is laying back on the carpet, he cannot see the impact zone. He has to protrude the barrel out the window to get on target.