I know there is a formula for determining whether or not to use your mileage to buy a ticket vs. the actual cost of the ticket, but I can't find it. Can anyone help me out with this? I need to go from DTW to AMS in July and the cost of the ticket is approx. $1200. Alternatively, I could burn up 100k ff miles from my NW account. Am I better off using the mileage for the ticket or just buying it outright? The fees and taxes on the award ticket amount to approx $75.