you would be right about at the top left edge of the map. Its impossible to actually read it. The gps also has a way to measure the actual path followed, but because of the loopback, you could be a mile and a half down the trail, and be a mile from the traihead in a straight line.

I will say thats a good idea not only to get a waypoint at the trailhead, but if you turn on the track back feature it makes a little line on the GPS showing your actual path followed. Its easy to follow the track back. As long as the new line being drawn on the screen is ontop of the coming in line, you;re going back the way you came.
Jim