Can't agree with you any less there, Pilot Pete. I for one use this method as an absolute backup:
Passing 20000 ft, shud hv 63 miles to run
15000 ft 50 nm
10000 ft 35 nm
5000 ft 20 nm at 250kt
If for instance at 50 nm to run I am at 16500 ft, I am 1500 ft high.
At any other level, I just add or subtract 3 miles per thousand feet. I wont be too far off...