If there is not a "400M to go" split or marker (I know there is in NY because I've run past it and the 800M to go split also), usually there is a almost always a 26.00 mile split or marker/sign to use. That's 0.21875 miles to the finish, easy to tell how fast someone closed in. I would divide the closing 0.21875 mile time by .875 (385 yards/440yards to the close) and get the last 440 yards pace. Then deduct the 0.4 seconds (400M vs 440 yards) assuming the 0.218745 miles were run at a 60 second pace. There may be a slightly shorter way to figure it out, but this is a simple, sure way to tell the closing split. And if there is a 42K mark, you can basically get the last 200M extracted as well.
NO need to stay on the track just because someone cant figure out or simplify closing speed on a marathon.
I was doing this in 1982.