First, the top end. You can buy miles for about 2 cents per mile, so subtract the taxes from the cost of the ticket, and divide by the miles you would have earned plus the miles required for the award ticket. If the answer is more than 2 cents per mile, you ought to be thinking about using miles.

Personally, my bottom end is one cent per mile. I will fly almost anywhere if I can do so for one cent for every mile I receive. Harder to do than it used to be.

In between the two is a judgement call.