Works the same as dollars per hour. As long as you don't try to convert it to dollars per hour you don't really worry about it. Make as many TFPs as you can for as little work as possible, then do the same next month.

When the union produces contract comparisons during negotiations they'll do the arithmetic and show you the conversion and how it compares to other airlines. It's kind of goofy but since it is based upon mileage, there actually are some benefits to TFP as leg lengths increase.