Interesting. Thank you!
However, I think there's a shortcoming in the software if you add only part of the extract to the smaller boil to keep the gravity correct. Hmmm....
There are a couple of things in play here. First, hops oils are limiting in solubility, and in isomerizing once the wort is saturated. That means that no matter how many hops you add to the wort, you're limited to under 100 IBUs or so just because of that principle. So the calculator may come up with 120, but it's simply a calculated number and not showing the very real ceiling of 100 IBUs. Actually, some of the hoppiest beers have been measured in actuality at 80 IBUs, even when calculated to 200+, such as Pliny the Elder. That's because of this solubility/hops oils saturation.
Also, when you do a partial boil of the wort, you are diluting the IBUs. Here's why. Say you did a boil of 2.5 gallons, using plenty of bittering hops and got an IBU of 100. Then you add 2.5 gallons of water at the end (with 0 IBUs). That means your five gallon batch has 50 IBUs at a max. That's the limitation of a partial boil.
It does help adding the bulk of the extract late in the boil- due to the wort gravity/hops oil isomerization issue but even so you'll never get the same IBUs as a full boil would, due to that dilution issue.
Adding the bulk of the extract late is one way to maximize the IBUs you'll get, though, and that should be done for that reason, as well as for better taste in the final beer (so you don't get that "cooked extract" taste that is present in some homebrews).
