I keep a screen grab of the "Mileage Estimator" with details expanded, for every purchase. One can see this before completing the purchase, so if one forgets or buys over the phone, one can duplicate the information. The word "Estimator" is troubling, but so far these grabs have never been wrong for me. (I too track everything in a spreadsheet. I once made United 1K by $38. That's my idea of cutting it close.)