Dear Mick, I fly these things, so hope I can help: It is less important where the descent is started than where it is finished! However an efficient descent [engines at idle or slightly above] starts around 3 times the height [1000's of ft] in nautical miles. Eg, Flight Level 330 = 33,000ft x 3 = 99nm or about 114 statute miles. Modern aircraft have a Flight Management Computer to crunch these numbers and take into account winds, temperature, weight, expected icing, atc etc. But the pilot still makes an educated guess to ensure he [or his first officer!] hasn't programmed something silly and to keep an eye on the pesky computers. The "steps" on descent are usually caused by atc: either terrain/airspace or most often traffic restrictions. To slow for landing the pilot configures the wing [flaps] in stages and lowers the wheels - you can feel/hear these events on arrival. A B747-400 normally approaches at around 160mph +/- depending on weight. ON takeoff, at max weight out of a high airport [eg Johannesburg] the aircraft may be lifedt off at the tyre speed limit = 225mph....