A 53s 400m is roughly equivalent to a 2:00 for a HS runner. A 53s for someone who has trained for a while has a chance for a 1:56 or 1:57. Typically its a good idea to go with the 4s rule which is 53 + 4 = 57 for first lap and 57 + 4 = 61s for second lap. Which is a 1:59.x under the best circumstances.
