In article ,
john smith wrote:


It believe it is based on the instrument requirement (?) of 500 fpm rate
of descent.

That's the absolute minimum descent rate; more typical is a 3-degree
(roughly 300 feet per nautical mile) descent. ATC is expecting a normal
rate of descent for your particular type of airplane to achieve that.

In a spamcan doing 90 knots groundspeed, 500fpm is about right. In a
typical jet doing 450 knots over the ground, that's going to be more
like 2300 feet per minute. (Groundspeed in knots * 5 will give you a
pretty good target to achieve a 3 degree descent.)