Flightsimmer or not, you're more than welcome to come in here and ask questions - so welcome. It's only when armchair pilots start telling us how it is that we'd have a right to get on the defensive.
I'm on my second aircraft type, and both types operate along very similar lines.
To keep it simple, a typical rate of decent that works well for everyone in your commercial jet, is approx 2000ft/min. So, if you were cruising at 36,000, you would need 18 mins to hit sea level.
Using 360kts ground speed, that works out at 6 miles a minute. So for every 2000ft you drop, your covering 6 miles. 6 x 18 = 108nm (very close to the 100 mile rule of thumb above).
I use this exact technique above, and it serves me very well.
You have to bear in mind however, that as you descend the wind varies, and most airspace in the UK requires you to be 250kts or less below FL100. So you have to slow up somewhere too.
Also, which the tree huggers are still reworking the airspace to enable Constant Descent Approaches (CDA), many STARS require you to descent early and fly level during the arrival (France and Germany are especially bad for this).
With regards to the FMS, they all do more or less the same thing. Most allow you to specify heights for the waypoints as required, and allow you specify the rate of descent or desired glidepath between the points. Using vertical nav on the flight director will then vary the decent as require to hit the altitudes at waypoints as required.
Lastly, to fly a 3º decent profile (ILS, NDB, VOR APPR), the rule of thumb is 5*groundspeed. For 100kts, thats 5*100=500ft/min.