I'm making Jamil's Belgian Dark Strong Ale from "Brewing Classic Styles", which calls for 4% Hallertauer. The Hallertauers I've got are only 2.1%, which is no biggie. But when I use Beer Smith to work out the appropriate amount of extra hops to add, I notice a big discrepancy between Beer Smith and Jamil's recipe. Jamil's recipe says 2.42oz of 4% Hallertauer give 31 IBUs, whereas Beer Smith says 2.42oz of the same hops give 21 IBUs. Anyone have any suggestions as to which I should go with?