The actual IBU level of a beer can only be measured in a laboratory. The BJCP guidelines provide ranges from actual laboratory data, not Rager or Tinseth formulas. That being said, Tinseth is the most accurate of all the formulas. And Rager is the one that a lot of people use largely because it's the one Jamil uses. Personally, I like accuracy so I use Tinseth. In the end, it doesn't matter much which formula you use as long as it's consistent. But if you want to compare against BJCP guidelines, etc., then I really can't help but recommend Tinseth, as Rager is WAY off the true IBU values for many beers.