I had the exact same question a few years ago and asked it on TechTalk. This is the response I got from David Houseman on the issue:
Why are you looking for formulae for IBUs in the BJCP style guidelines?
There are none. No formula calculations. The IBUs for the styles are
they are, independent of how someone arrives at them. These weren't
calculated but rather are those that are historically, traditionally or
exist today in the best examples of the styles. If you need formulae for
you to use in figuring out how to achieve those IBU levels in your system
then look at ProMash or a number of texts. Ray Daniels' book, Designing
Great Beers, is a great read for looking at the ration of IBUs to starting
and finishing gravity.
Thanks for the prompt reply.
I guess I'm having trouble correlating the IBUs in the BJCP texts to the IBU
formulas. We do use BeerSmith, Promash and BeerAlchemy (Mac program) but
were unsure which formula would most closely align with the IBU ranges.
For example, we were designing a beer over the weekend that we have
previously made. We wanted to reduce the hop bitterness from 100 IBUs to 75
IBUs. We normally use Tinseth (74.5 IBU) to calculate IBU. Daniels isn't
that far off Tinseth (76.6 IBU) at our volumes, but Rager (96.6 IBU) and
Garetz (63.9 IBU) are much different in their calculations.
If IBUs are subjective, then it just may be a matter of buying commercial
examples of a style, tasting them, and brewing test batches of beer to
determine which IBU calculation matches our palate.
I was just wondering if there was a correlation between the IBU calculations
and the BJCP values.
Thanks again for your reply.
The simple answer is no. What the BJCP is telling you is the appropriate
range of IBUs in a specific style. How you get to that point isn't a BJCP
concern. Use any formula that works for you to get to say 75 IBUs. I use
Rager in Promash but have always had similar questions about which may be
more accurate. That would be a good point for research. But 75 IBUs is