3 x height in whole thousands of feet, + 10 nm for deceleration from high speed for jets, +/- 1 nm per 10 kts head/tailwind. Therefore, FL 330 in 50 kts tail = 3 x 33 + 10 + 5 = 114 nm descent point from runway threshold. Right, now where's my answer about takeoff alternates; I need to go for a coffee!