Whilst you cannot calibrate the numbers that appear on the LibreLink app (or Libre Reader), if you do a daily finger prick you can get a general feeling of how close the Interstital Fluid Reading calculation (done by Libre) is to a blood glucose reading.
For example, if you find finger pricks (taken when your levels are stable and between 4 and 9) are always around 1.5 units higher than Libre at that time, you know to add 1.5 to what you see on Libre.