Maybe I'm remembering things wrong, but isn't the bitterness of your beer affected by the amount of malt in my grist? So if I were to go from having 10 lbs of pale malt to 15 lbs, shouldn't I see my bitterness go down?
Yeah, the algorithms can be a little difficult to choose since most of us don't know the actual IBU's of our beer. I choose to use the Tinseth algorithm and include the gravity correction for full volume boils, and that seems to get me close.