At the January 1924 meeting of the FAI it was agreed that over the 3km run the aircraft had to be flying at a height of 50m above the marker points and also had to be flying at that height for 500m before and 500m after the run (I cannot remember how much leeway there was in that). The aircraft could not exceed 400m during the entire flight, so it was still possible to dive down from 400m to 50m prior to entering the 500m pre-course part. The rules were tweaked bit by bit over the following years as speeds increased, as seaplanes took the record and so on.