IBU calculations

Just installed. Everything seems pretty straightforward. However, I am making a recipe, and started with something off the web that for example states boiling half an ounce of Centennial for 20 minutes would yield 6.4 IBUs. That seems reasonable to me. When I enter this into BeerSmith it calculates it as 20.5 IBUs. Something is way off either the recipe or my software is not set up.

Do I need to configure anything beyond my equipment profile to have good numbers for IBU calculations?

It's going to depend on which formula you're using, what your final volume is and what AA% you have. All of those have to match the original recipe. It's typically best to enter the recipe with equipment that matches the original brewer, then use the Scale Recipe function to fit it to your system.

I plugged a half ounce of 10% Centennial into a random 5 gallon recipe and got 15 IBU, using Tinseth. It didn't come down to 6 until the batch size was 15 gallons. At 5 gallons, the Garetz formula still showed 9 IBU.