Perhaps i've missed something here but I was always taught the 3 x height to lose method. The rate of descent required to achieve the 3/1 profile was 5 x groundspeed.
So, turboprop at 300 kts g/s descending from 20k ft to 5k ft = 3 x 15 which equals 45 miles. Rate of descent is 5 x 300 = 1500 fpm
Cessna/piper travelling at 120 kts at 5k feet descending to 1k feet = 3 x 4 which equals 12 miles. rate of descent is 5 x 120 = 600 fpm.
For those that want convincing, then go work it out with a calculator, it works out prety much on the nose.