Using this rule, in light twins anyway, usually do 3xaltitude to lose plus 5nm. Rate of descent is 5x groundspeed.
eg Descending from 10,000ft to 7,000ft at g/s of 140kts
10-7= 3, x 3 =9nm,+5nm =14nm therfore descent would start at 14nm out.
R.o.D =g/s x 5= 140x5 = 700fpm