When does your presentation start?
As soon as anyone from the audience can see you.
If there are people in the room as you are setting up your PowerPoint, you’re on. If your class starts to assemble while you’re putting handouts on the table, you’re on. If you have to enter and cross the room to a certain place before you start talking, you’re on as soon as you enter the door.
Think of it like being the host at a party. If you’re still putting out the hors d’oeuvres when your prompt guests get there, you welcome them in with a big smile and keep getting ready. When you’re the one in charge of the room, you don’t suddenly become that person when the clock strikes the minute you’re supposed to start.
You are making an impression from the moment they see you. Are you harried? Are you stand-offish? Do you seem happy to be there? Are you in charge of the room?