The difference in height between the top and bottom altitudes given is used along with the Slope angle to compute the run (distance) from which to start the descent, i.e. the Top of Descent distance. Any offset given (>0) is also added, for instance when it is desired to be at the bottom of descent at this offset from the destination.
Quick rule-of-thumb for mentally calculating Top of Descent for a standard 3° glidepath slope
The Top of Descent can be roughly approximated by a general quick rule-of-thumb:
(Difference in Height/1000) x 3
NB: This rule-of-thumb only works for a 3° glidepath (standard slope).
How Descent Rate is computed
The computer firstly uses the Wind Correction Angle (WCA) to calculate the Ground Speed (GS). Secondly, the Ground Speed is adjusted accordingly for the wind i.e. subtracting any headwind or adding any tailwind. Thirdly, the revised Ground Speed and the Slope are used to determine the Rate of Descent. Finally, the Rate of Descent (given in ft/min, fpm) is calculated from the tangent of the slope and the True Airspeed. The Wind Correction Angle (WCA) and resultant heading to fly are also given for convenience.
Quick rule-of-thumb for mentally calculating Rate of Descent for a standard 3° glidepath slope
The Rate of Descent for a 3° glidepath (standard slope) can be roughly approximated by a general quick rule-of-thumb:
(Ground Speed / 2) x 10
For more accuracy, adjust the Ground Speed for any head or tailwind before using the above rule, as follows:
Subtract any headwind component from the Ground Speed Add any tailwind component to the Ground Speed
Note: This rule-of-thumb only works for a 3° glidepath (standard slope).