#1 - Multiply the no. of thousands of feet to
lose by 3. Groundspeed / 2 * 10 will give you your required rate of decent
for a 3 degree glide slope.
example: FL350 to FL100 => 25,000 ft down
25 x 3 = 75, so start at 75 nm
GS = 320 kts ? => 320 / 2 * 10 = 1600 => 1600 fpm is your desired rate
#2 - Start descent when time to airport equals altitude to lose for 1000 ft/min
example: 10,000 feet to lose, start descent 10 minutes out at 1000 ft/min
#3 - Divide the difference between Cruise Altitude and Descent Altitude by
the Rate of Descent. That value is your Time to descend. Multiply this by your
Groundspeed in Cruise and divide the result by 60. The result is the DME prior
to which you need to begin your descent.
example: FL360 to 6000' = 30,000' /1500'per minute = 20 minutes.
Now, (20x420 Knots G/S)/60 = 140 nm from the desired point.