It depends on the particular airplane, weight, ias, wind...
I use FLx3 plus 10 (only plus 6 below 10000) for typical weights and speeds, and when I am in VNAV, i find that rule to be very accurate, specially at lower levels where weight or speed or wind make less difference.
When on the magenta donut, with 8000 i always have about 30 to go, with 6000 always 24... And so on. Just as my rule says