I brewed a Munich Helles from “Jamil’s Brewing Classic Styles” which calls for 11 pounds of grain (10# pilsner, .75# munich,.25# melanoidin). His recipes call for 6 gallons left in the boil kettle. The hop addition is 1.1 oz. Hallertau 4.0 % for 60 minutes. He shows this to yield 18.1 IBU.
Plug this into Beersmith and it gives me a value of 13.6 IBU. Most of his other recipes i have brewed were pretty consistent with values from Beersmith but this seems too far off.
Has anyone else had similar experience or are my calculations off?