Descent planning III:
When performing a CDA, ATC gives you "Trackmiles" to go:
groudspeed : 60 = X,
trackmiles : X = Y,
current altitude : Y = descent rate.
For example, assume a groundspeed of 360 kts at FL150 and you have 30 NM till touchdown;
360 kts : 60 = 6 NM/min,
30 NM : 6 Nm/min = 10 min,
FL150 : 10min = 1500 fpm.
(Adjust for changes in groundspeed off course)