To calculate the vertical speed required to reach a destination with a given groundspeed and distance, use the following:
Vertical speed required (fpm) = (altitude to lose (ft) x groundspeed (kts)) / (60 x distance to go (nm))
To calculate the distance required for descending at a particular vertical speed & groundspeed, use the following formula:
60 x descent (fpm) / groudspeed (kts) = ft/nm
altitude to loose (ft) / ft/nm = distance required
aka: altitude to loose (ft) / (60 x vertical speed (fpm)) = distance required (nm)
Another way to look at is: if you need to loose 24,000ft, descending at 2000 ft/min, it will take 12 mins (24,000 / 2,000 = 12). With a groundspeed of 240 kts, it equates to 4 kts per min (240 / 60 = 4). 4 (kts per min) x 12 (mins) = 48 nm (distance required).
Thanks to:
http://forums.flightsim.com/vbfs/showthread.php?27140-Rate-of-Descent-Calculation
http://forums.vatsim.net/viewtopic.php?f=8&t=6861
http://www.atpforum.co.uk/showthread.php?t=9907