How about 800M? You just double a 400's result (how much time it took) and that is going to be pretty fare because 800M is more exhausting than 400M but need to have 2 starts compensates the difference.
I dont think a 400M runner can keep up the same pace over 800M because of the oxygen debt that builds up in the muscles. So you cant just double their 400M time to get their expected 800M time.