while it is old advice...a long , idle , descent can lead to a surprise if you need the engines ...so, check them , by revving them up a bit, at least by 3000'. Indeed, it was our policy to NOT be unspooled down low.
altitude to lose
times 3nm per thousand feet = distance to lose altitude
5 times ground speed equals rate of descent in feet per minute to make good above.
but, because NO one knows how the winds will change as you descend, start down 10 miles before the above equation...and in this way you will be able to gradually level off for a smooth ride...otherwise the descent level off will break your neck (kidding).
so...lose 10,000 feet
u need 30 nm
400 knots ground speed means 2000 fpm
so start down 40 miles prior to crossing fix and adjust rate of descent as ground speed changes
good luck