There are two different methods used to calculate hop bitterness. The default IBU calculation uses the Rager method, from Norm Pyle's "Hops FAQ" <http://realbeer.com/hops/FAQ.html>. The other method is the Tinseth method, from Glen Tinseth's "Hop Page" <http://realbeer.com/hops/>. To change to the Tinseth method, open up the Configure... dialog and check the Use Tinseth bitterness calculation box. The utilization table for the Rager method can be edited by changing the appropriate area of the qbrewdata file.

Please see the above links for details on these formulae, as they can be a bit complex.