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.

Keep in mind that UA will always allow you to upgrade yourself with 500s, even if you don't have enough ecerts in your account. Why? Because you can purchase them at the counter to support the upgrade. I watched a guy checking in once who found this out the hard way. He thought that he had upgraded himself on line for free.