CoolP wrote on Nov 28^th, 2011 at 1:54am:

...Speaking of a 3 degree descent by the rule of thumb, you e. g. would be at FL300 and 90nm out to come in at 0ft. You can add some 10% buffer zone and start your descent at 99nm out, so you can counteract some wind influences. The trick is to check your descent rate on a regular basis, so don't just start the descent and wish for some luck, but e. g. check it at FL200, 100 and so on. You have to alter your vertical speed either way, which is what a VNAV system does all the time...

This rule of thumb I've learned a long time ago with a friend of mine who was a real life airline pilot. I always use it and it works very well for me (FS purpose). Is the 3x rule and is very easy to use. Just divide your cruise altitude by 3 and the result will be the TOD. For example, if you are flying at FL 300 and the destination airport is at 0 ft, you must begin your descent between 100/90 miles - TOD = [(300-0)/3] - away from your destination, using a descent rate of 1,800 ft/min and IAS around 310/300 knots. If you feel you are descending too fast, reduce the descent rate and/or speed. Remember that below 10,000 ft, IAS must be kept at 250 knots or less, and don't forget to look constantly at your wind vector. Whatever, try to adapt the rule to the aircraft type you are flying.

Pinatubo.