In a sense, the area shown on a presentation screen is a stage. Just as on a theater stage, you do not want to get too close to the edge. In this case you won't physically fall off the stage, but your words and images might. Leave about 15% of the edge of the area as a safe margin. The computer screen is not exactly the same aspect ratio as a data projector or television screen. Sometimes different projection methods do not show all the area you see on a computer screen.
