Try this for the descent and distance.
Distance To Descend 1. Take your altitude and multiply it by 3. That equals your distance in miles to begin your descent. 2. Now take half your ground speed. This is your rate of descent in hundreds of feet.
Ex. If you are flying at 12000ft at a ground speed of 150kts and you need to descend to 2000ft, the difference is 10000ft. Multiply 10*3=30 miles out you must begin your descent. Half your ground speed is 75, add a zero, and 750 ft per minute is your rate of descent.
The distance to descend at 500 ft per minute. Equals 2 times the ground speed in miles-per-minute times the altitude to lose in thousands of feet.
Ex. If ground speed in the descent is 155kts/180mph(3 miles-per-minute) and you must descend 8000ft, you must begin you descent 48 miles from the point at which you must be at the lower altitude. 2 times your 3 miles per minute ground speed equals 6 times 8 (your altitude to be lost in thousands) equals 48 miles.
