Well, my MS Flight Sim manual tells me to use the "3-to-1 rule": take your altitude in feet, drop all the zeros and multiply by 3. The result is the distance in nautical miles from your destination where you start a descent at 250 knots and 1,500-2,000 feet per minute, with thrust set at idle.
i.e. for FL270: 27x3=81 nautical miles from your destination.
Obviously, to state more accurately other considerations need to me made e.g. type, acquired descent rate, speed. However, i'm sure the above calculation would suffice as an 'off the cuff' answer...
Or maybe not?
Ed.