From a UK military point of view, unless it has changed in the last 16 months, it is (was) forbidden to anticipate rates of descent. You are (were) only permitted to issue a descent instruction based on the ACTUAL position of the aircraft with reference to the radar vector chart. Therefore in your example , flying finn, the aircraft would have been descended to 5000ft QFE (as that is what the RVC is based on) or equivalent until the aircraft crosses the 20nm range towards the aerodrome.