Descent planning:-
height to lose x 3 = distance to top of descent, then add 5nm to slow a/c and configure.
Rate of descent = 5 x groundspeed
Example - 20,000ft descent to 3000ft at 200knots =
20 - 3 = 17, x3 = 51nm, add 5nm to slow a/c = 56nm
5 x 200 = 1000ft per minute
Oh and for Final 3 greens post - perhaps easier just to work out 5 x groundspeed = rate of descent for 3 degree ILS glidepath