Go to grand slam stringers . com (no spaces) and get the calibrator they have on there. I believe it runs for about $39. It works great and I believe it is used widely throughout many stringing rooms on tour.
Edit: My apologies but that calibrator is actually no longer on that website. Anyway it is called the UltraSport 50 by J Scale.
Here it is even cheaper than GSS had it when I bought mine: