well i just brewed this beer on monday night. the OG was 1.050. hopping schedule was:
7AAU centennial, first wort, 90min boil
2.6 AAU crystal, first wort, 90min boil
.875 AAU cascade, 20min
2.3 AAU centennial, 20min
.875 AAU cascade, 5min
2.3AAU centennial, 5min
it's made its way down to 1.012 by last night (there aren't many things about brewing i love more than going from pitching to final gravity in 72 hours, btw!) and i'm having another taste as i write this. beersmith says 40.9 IBU, and i used the first wort hop setting, which increased the IBUs for those hops by about 10%. this isn't a 41 IBU beer. it's a tasty beer for sure, and it's very well balanced, but i don't think it would be if it were 41 IBU. i don't have much of an internal scale for IBUs, but presuming other beers i've made were at the bitterness levels that beersmith predicted then i'd have to say this one is at 25-30. the ONLY thing i can think of as a reason that it might be higher then it tastes is because the stuff i've read about first wort hopping says that taste tests revealed that a beer brewed with first wort hops had a lower perceived bitterness than the same recipe brewed to the same bitterness with out the first wort hopping. in other words, i made a tasty beer, but whether some AA was lost due to first using the hops as dry hops is unknown.
i think this problem deserves a 1gallon side by side experiment. i'll mash enough grain to make two 1 gallon batches the first will get first wort hops using fresh hops, the second will get an identical amount of first wort hops that were previously used as dry hops. i suppose i'll have to make a 1 gallon batch of beer just to dry hop so that the proportions are right. i'll use only 1 type of hop and i won't add any aroma or flavor hops to eliminate variables. question/comments/thoughts very appreciated!