After reading another thread regarding the differences in full boil vs. partial boil and the effect on IBU's, I downloaded and started playing with the IBU estimations. It is clear there is a significant impact in going from a partial to a full boil that would explain why we have brewed beers that were significantly more bitter than we expected.
One thing I noted was the difference between the calculated IBU's in a custom entered recipe vs. the predicted IBU's derived from the Tools/Hops Bitterness menu option. In my instance the recipe calculated 14.8 IBU's, but the same info entered into the hops bitterness panel returns 17.8 total IBU's. Why the difference?
Both predict similar changes in the bittering as the boil volume is changed, but I would have expected more correlation.
7 lbs pale LME
1 oz Cascade bittering hops 60 mins
1 oz Williamette aroma hops 5 mins
5 gal batch, 3.25 gal boil