On the ND(Navigation display), that open green circle (deceleration point), shows where the aircraft will start decelerating to the speed which is shown in bold on the legs to page.If you also refer to the descent page,you will also see the position referred to, with a display showing the "hard" speed and the flap setting required.
The more accurate the wind direction, speed forecasts, and ISA deviations input on the descent page, the more precise the FMS will maintain the desired flight paths, and speeds.
LIke I mentioned earlier, for the computer, it's "garbage in,garbage out".