To Memo -Unfortunately your method is incorrect. The fuel and time savings have to include the additional miles you flew to get to the closer TOD for a 330kts descent. You're much closer to the airport, by more than 20-30 n.m., or more, if you can descend at 330 kts vs. 250 kts. Getting closer cost you time and fuel.A better way would be to figure out what the time and fuel was from a fixed point, say 100 n.m. to 10,000. Try the descent at both 250kts and 330kts.That said, if you're trying to save time the descent page is the most efficient way to save time vs. additional fuel burned.