How low do you let your tank get before stopping?

If I have confidence in the miles to empty reading I use Google or Garmin to search for the last appealing gas station withing that range and head for it. Unfortunately, my pickup does a terrible job of adjusting expected mpg for current load condition so its predictions have to be taken with a whole bunch of salt. I don't understand why Ford has you identify each trailer that you have then doesn't do anything useful with that information (such as adjust predicted mpg).