In my VC10 I will often fly a CDA so that I am well prepared if it is required, eg at Hannover or Edinburgh.
My rules of thumb are to be at 6000 feet with 20 nm to touchdown, providing I am on speed (210 kts). It will normally be idle power down to about 2500 - 3000 feet, then increase the power to counter the drag changes during configuration, the power will vary depending on weight/temp/wind etc, to stabalise at approach speed (170 kts). Obviously, in the VC10 I have no advanced automation, so the whole thing is judged mentally and constantly reviewed.