Keep in mind that the 400 feet third segment is gross while terrain calculations are based on net.
If you play with the geometry, 400 feet gross, for a simple case, gives you something in the vicinity of 280 feet net for a twin (according to my back of a fag packet calculation). If you start pushing down the 400 feet gross, you end up with a need for very little in the way of terrain to make the thing work.
At the end of the day, it's just a line in the sand sort of number.