Does that help? http://www.flightsimaviation.com/rule-of-thumb/11_Descending_Distance_and_Rate_of_Descent_RoD.html
Lou also wrote something on that topic.

The ATC guys have their traffic, control zone and responsibility restrictions, but apart from that, they tend to talk you down around that 3 degree reference line. Unless you are flying a very special plane and setup, you may follow this line. The above rule of thumb describes just that.

However, the ATC isn't a reference for flight planning, so the responsibly to descend (or asking for it) timely and sufficient for your plane's need is the pilot's job. If there was weather on or in the way, the same applies. The communication then sets the marks in controlled airspace.

ATC sees your plane type and GS (groundspeed), so they can do the maths of course (or use software) and see when a nice 3 degree descent path is available for you,  and if it would fit into their plans of seeing you higher/lower at certain waypoints of your flight plan. So you are not always descending towards a destination airport, but also to a certain waypoint and stop your descent there. That's where the formula comes in handy.

You, in the plane, have to check your descent rate against the altered groundspeed from time to time, otherwise you will be too fast on the way down. The descent itself (into lower altitudes) and some changing winds may affect it more or less severely, but in general, your groundspeed gets less when descending, so your vertical speed has to be adjusted and can't remain steady.
That's actually what the VNAV does, including hitting the target (step) altitudes at certain waypoints of the arrival procedure. Yes, it even adds some 'optimal value' calcs to allow idle descents and also optimum speeds on the way up, but that's too fancy stuff for us flight simmers when using FMC-less planes.
However, if you're good at basic trigonometry, you can add it and aim for the speeds mentioned in the plane's manual.

But working that basic way easily allows for flying with online ATC for example. And you won't miss the FSX one at all.  ;)