Originally Posted by marco

You can also do the mental math another way if you like to hang at your cruise altitude until the last second and know the performance of your airplane and the time that you should cross the fix:
a. determine altitude to lose
b. determine the descent rate you want to lose that altitude to make the crossing and convert to minutes
c. start your descent when you are that many minutes away from the fix

ex: cruise altitude is FL310, crossing restriction is 35 out of MHT at 11000 = 20000 feet to lose. I want to descend at 5000' per minute: 20/5=4 minutes. -Start my descent 4 minutes out.
-Cruising at 400kts is roughly 7 miles per minute, multiply by descent time: 7x4=28 and add to 35 (crossing fix): 28+35=63. (round up for fudge)
-Start descending at 65 miles out of MHT at 5000'/min maintain your cruise airspeed.

Now if ATC gives you an airspeed restriction while you're in the dive, you're not gonna make the altitude at the fix. Maybe I do need pen and paper for this one!!