They were trying to establish statistical margins for a range of aircraft
That was the gist of the story given to me by John Walshe (ops eng boss - absolutely lovely bloke) at Ansett in the late 60s .. pity he didn't give me the document to read in toto.
approximate the curved path by two straight lines and guess what? the convenient break point was 400 ft.
ah, now that I didn't know .. thank you