What I'm really interested in is where on earth the idea came from that flap retraction is linked to height.
Historically this comes from airport design criteria which were matched to aircraft design. Around a round table (many round tables over the years) starting in the 1930ies, representaives of towns and aerodromes and representatives of aircraft manufacturers debated over the obstacle-free zones around aerodromes and the climb performance of aircraft. The aerodrome managers wanted to build as close to town centres as possible, the aircraft manufacturers wanted to use the cheapest engines on the market, and eventually that 400ft compromise evolved that found it's way into both aerodrome layout and part-25-design criteria. If someone wants to do a historical search, I'm pretty sure that 400ft-obstacle which was the origin of all this can still be identified somewhere. Some building around Idlewild or Tempelhof, who knows...
But where on earth this 1500ft-figure came from is a miracle to me. Totally arbitrary, nonsensical, yet part of many training programs and SOPs. Personally, I tend to disregeard it with the exception of my yearly checkride at FSI.