Sonicbum,
You said nothing new here.
Yes the speed at high levels and high weights goes down rapidly, meaning that OP DES should be selected earlier than later. However the aim is fly level for as long as the aircraft performance allows in order to stretch the descent. Hence pulling OP DES indiscriminately for obstacle strategy, regardless of the energy of the a/c, is not the way to go.
Reaching green dot is indeed when the speed trend touches green dot.
Thank you.