altitude to lose (in thousands) x 3 = dme to start down

G/S divided by 2, add a 0 = FPM required for 3:1 calculation. (also works for FPM required for glideslopes)

example: you are 24000 need to cross nesto at 10000. Groundspeed is 420

14 x 3 = 42nm

420 / 2 = 210 add zero 2,100 FPM

start down at 42nm and use 2100 fpm

make sure to increase descent rate when GS increases