df1, you are right this is not accurate enough, but it is also too accurate (
). The given ROD and speed need to be used, but the deceleration distance is ignored.
There are two typical, similar questions: (i) giving altitude at top and bottom, rate of descent and some measure of mean groundspeed, asking for distance taken to descend and (ii) giving altitude at top and bottom, some measure of mean groundspeed and distance available, asking for rate of descent.
In both cases the key is time in the descent.
(i) find the time by dividing the height to be lost (make sure the pressure reference is the same) and divide by ROD.
Multiply mean groundspeed by time to find distance.
Note: if the question asks you to be level some distance before the beacon, don't forget to add that distance
(ii) find the time by dividing the distance available by mean groundspeed.
Divide height to be lost by time to give rate of descent.
You will notice that each case is simply two speed - time - distance calculations (rate of descent is just a speed, downward, altitude change is just a distance). Time = distance ÷ speed followed by either distance = speed × time or speed = distance ÷ time. Time is the key: the movement across the ground takes place in the same time as the descent.
Good Luck
Send Clowns
Navigation Instructor, BCFT