Thanks Schemy and Randy_Bugger for the replies! This is really helping me to learn a lot and think about new stuff
I'm going to try to tweak the recipe a bit without beersmith. I don't have a copy, although I know there is a free trial available. I learn better by doing things by hand anyway (I'm a math teacher after all). It seems like IBU calculations for whirlpooling are pretty undecided, so beersmith might not be that much help anyway.
I think you are right that the hop values are a little low, but perhaps not too far off if I can whirlpool for 10 or 15 minutes and get some IBUs out of the last batch of hops.
Here's my calculations and some changes:
INCREASE Magnum hops to 1oz. (previously 0.76 oz)
(1oz * 27 efficiency * 15.1AA) / 6.7 batch size = 60.85 IBU
INCREASE Chinook hops to 1oz (previously 0.95 oz) and change the time to 20 minutes (previously 15)
(1oz * 12 efficiency * 11.4AA) / 6.7 batch size = 20.4 IBU
That gives me a total of 81.25 IBUs, leaving 11.75 more needed.
Like I said, it seems that IBU calculations for whirlpooling are all over the place. However, if I whirlpool for even 15 minutes I think I should be able to get at least 2% efficiency out of the final three hops selections (I'm open to being corrected on this).
This would give me 13.18 more IBUs. For a total of 94.4 IBU, which is close to the 93 in the real deal DuganA. Even if I only got 1% out of 'em I'd be within 5 IBUs, which wouldn't be very noticeable.
So, here's the revised recipe (version 2):
LME - 9.8#
Dark Aromatic - 0.5#
C-120 - 0.2#
60min - Magnum (15.1%AA) - 1 oz
20min - Chinook (11.4%AA) - 1 oz
0min - Columbus (12.0%AA) - 1oz
0min - Centennial (9.4%AA) - 1oz
0min - Chinook (11.4%AA) - 2oz
Whirlpooling the last three for 15 minutes.
Dry-Hop Addition (needs multiple sessions??)
Chinook - 3.78oz
Columbus - 3.78oz
Yeast - California
Ferm Temp - 68F