Originally Posted by WayMaker

The advise from wxguy and collegeflyer is correct. But let me add this note:

You will find many UA agents unaware of how it works: as-the-crow-flies, from origin to destination. Be ready to patiently explain it. I had an agent try to force the system to charge me more 500s some years ago. I flew ATL-ORD-AUS in F using just two 500s since AUS-ATL is 813 miles. But it took a discussion at check-in with an incredulous, less than fully informed agent, who was quite adamant that I owed UA four 500s. A supervisor reluctantly cleared up the discussion. That was some years ago. Perhaps the system can't be overridden now.