A common descent profile uses a 3 degree flight path. It means a descent of 318 ft for each nautical mile travelled forward.
A good approximation, to simplify the calculations, is to aim for a descent of 300 ft for each nautical mile. This is actually a 2.82 degree profile. Thus, the distance D, as in the figure above, can be calculated dividing the height one needs to descend, expressed in FL, by 3. For example, if one needs to descend 10000ft; D = 100/3 = 33,3 nm.
Some folks prefer to estimate the distance D by multiplying the height to descend, in thousands of feet, by 3. That's fine too; this actually corresponds to a 3.14 degree profile. Using the example above: D = 10 x 3 = 30 nm.
To estimate the rate of descent (ROD) multiply the groundspeed (GS) by 5. For example, for a GS of 120 kt, the required ROD is about 600 ft/min.
Note that, the groundspeed is likely to vary with the descent. There are two reasons: first the TAS usually decreases for a constant IAS descent. Second, the wind usually varies with altitude (direction and speed). So, if there is a significant variation in the GS, the ROD needs to be adjusted if a 3 degree descent profile is to be maintained. Keep an eye on that GS!
The mobile application allows you to explore the descent profile and the relationship between height, distance, groundspeed, angle and gradient of descent and time of descent. Have fun!