I have flown these thing for coming on seven years and that experience has taught me NOT to use the FMS for controlling your speed, especially below 10,000'. It's like having a random speed generator working against you all the time. Basically, it's rubbish because it is badly designed and written by idiots who don't fly. You are better off creating your own waypoints and manually selecting speeds at these points.
Remember that you can create distance arcs simply from the NAV - FIX INFO page. You enter a waypoint ident (including ILS or Navaid), omitting the bearing and just entering a distance an are/circle will be drawn on the NAV screen. If you make this arc at say 35 mikes, hit it at 270-280 knots and then select 250 knots and FLCH. Make another arc on the same page at 15 miles. Pass that one at 3,500 - 4,000' at 200 knots and select FLCH. Assuming you haven't got a nasty tailwind, you are in. If you so wish you can also turn the intersections of the arcs with your track into waypoints; just by pressing the line select key to the left of the arc twice.
Always remember, this thing will generally NOT decelerate from 250 knots down a three glide slope when clean. You really have to use the gear, brakes and possibly flaps. And basically, use them earlier rather than later.
Hope this helps.