What should you do if a stock you are holding goes up 50% in one day?  Yesterday night at 5:30 I found myself contemplating this happy question on ARTX (Aerotech, Corporation), which had gone up 50% on news of a large order from the U.S. Department of Defense.  So, I did what I usually do when confronted with such questions — I used Equities Lab to get an answer.  …