If it's like the 777 the FMC target speed is just that.
There is an energy conservation logic built in to the AT system, which allows a window of speed before the thrust is adjusted, just in case it doesn't have to.
For the NAT we enter the cleared mach # in the FMC and then for the exit point pre-select ECON. That's easy.
Also on descent using FLCH instead of VNAV with the speed window open (or revising the descent speed in the FMC) will get quicker speed adjustments in my observations.
halas