There's no single formula since every person's situation is different. There have been lots of threads on this topic here and they all come down to that.

You're contemplating a use that comes out to about 1.36 cents of value per mile. Many would consider that too low, especially since a paid ticket would earn you about 8,000 miles so the award really costs you 48,000, not 40,000. These people would buy the ticket and save the miles for a future use. (All these figures are per ticket. Double them for two.)

That, however, assumes they have a future use, and that this potential future use is worth more to them than $546 in their pocket now. How do you feel about the relative worth of 48,000 miles in the bank versus $546 in cash? If someone offered you $546 for the miles, assuming it wasn't in violation of program rules and all that, would you take it?

I'm flying BOS-LHR round trip in a few days. I got an inexpensive ticket but will upgrade. If you know how much business class costs, you know this is a value of much more than 1.36 cents per mile. But I would never pay the business class fare, so what is the "real" value of the miles to me? Hard to say.

On another occasion, I used 25,000 miles for a last-minute Boston-California round trip with no Saturday night stay. A ticket, which that time I would have bought if necessary, would have cost over $2,000. This is a real value of 8+ cents/mile. Unfortunately, clear-cut cases like that don't come along often enough to use up all my miles.

Many of us, myself included, also enjoy giving tickets to people. It's hard to put a value on this use. Some would call it priceless. Those who do it, though, tend to be frequent flyers with lots of miles.

If it were me, I'd buy the ticket, but it's not me. You'll probably get lots more opinions here. It's not them either. It's you. Good luck - and enjoy your trip!