Lately I've changed my IBU calculations to include Gravity Factor (Beer Bigness) and utilization. I got my information from John Palmer's How to Brew web page chapter 5.5 ( http://www.howtobrew.com/section1/chapter5-5.html
In his test case a beer with an OG of 1.080 has a total of 31 IBU's.
1.5 oz of 6.4% AA Perle hops (60 minutes)
1 oz of 4.6% AA Liberty hops (15 minutes)
Now I just installed a copy of Beer Smith and it comes up with 38.1 IBU's for this same recipe.
Is someone wrong, or is IBU calculation more of an art than a science.