code: protected static Double calculateDistance(int txPower, double bestRssiAvailable) {
          if (Beacon.getDistanceCalculator() != null) {
              return Beacon.getDistanceCalculator().calculateDistance(txPower, bestRssiAvailable);
          }
          else {
              LogManager.e(TAG, "Distance calculator not set.  Distance will bet set to -1");
              return -1.0;
          }
      }
docs: Estimate the distance to the beacon using the DistanceCalculator set on this class.  If no
  DistanceCalculator has been set, return -1 as the distance.
  @see org.altbeacon.beacon.distance.DistanceCalculator
  
  @param txPower
  @param bestRssiAvailable
  @return