My own rules-of-thumb are to
- value miles @ 1.5 cents when acquiring, though this is a rough guide. I generally won't spend more than 1 cent in their acquisition.
- upgrade international tickets with miles as best option, redeem for international premium cabin travel as second best option
But I currently have 2 tickets on hold with AA miles that represent 9 cents/mile... LAX-PPT-AKL-SYD-MEL-LAX in F on TN & QF, each ticket costs 145k miles and prices out @ US$13k.