Three years ago, I was 300 miles short for Prem Exec (United). I sent a simple letter specifying the situation to Mileage Plus in mid/Dec. When I called the Mileage Plus desk at end/Dec, they confirmed the Prem Exec status for the next year. Only 21 miles short? I would say you should be all right, but it does not hurt sending a little letter to them (I don't know which airline though).