There are obviously a lot of different ways to do this. I prefer this simple mental math method:

Altitude to loose X 3 = Distance to start descent

Ground Speed X 6 = Foot/min descent

Example.

@ FL 250 you need to descend to 15,000. Altitude to loose = 10. 10X3= 30. Start the descent 30 miles out.

Assume grounspeed is 300kts. 300X6= 1800 fpm descent.