Get the red as far outboard as possible. People are used to seeing red lights/reflectors on the back of a vehicle which help to outline its dimensions. I'd skip the white tape altogether if possible; white tends to be a colour used on the front of vehicles.
Check out the Versahaul website if you haven't already. Most of their carriers seem to come with red reflective tape already installed on the outer ends of the tray. They also offer a light kit which wouldn't be a bad thing if you tend to drive with the carrier on at night a lot.