Maybe the emails below will be of some interest. They are from Matt Van Wyk of Oakshire Brewing in Eugene, Oregon, and Vinnie Cilurzo of Russian River Brewing. I asked them what percent of the total IBUs they try to derive from the bittering addition in an AIPA. I also put the question to Mike "Tasty" McDole, and Denny Conn. (I don't think they would mind my sharing their info here). Draw your own conclusions.
It depends. It depends on how bitter you want the beer and how hop flavored and aromatic you want it. That, in many senses is up to personal taste. If you want to follow a 1:1 BU:GU ration, a 1.066 beer would be 66:66 or 66 BUs for a beer with 66 gravity units.
In my opinion, IBUs are not important and I wish they didn't appear on our bottle. Experiment with different levels and see what you like better. at 5-10 gallons per batch, you can do several experiments. Good luck!
Thanks for the email, no, we don't have a set percent of BU's we want to get out of the first hop addition. It really varies from recipe to recipe. Since we make so many IPA's, I mix it up so some get a bunch of bitterness from the initial addition, while others get a bunch of bittering from the middle addition or even the last addition and thus those beers have a bigger hop flavor and aroma.
I hope this helps,
I try to get 2/3 of the IBUs in the last 20 minutes of the boil (and post boil if there's a flameout addition). So the answer to your question is I try to limit the 60 minute addition to 1/3 of the total IBU's.
...I start by deciding on a rough BU:GU ratio for the beer. Then I add enough FWH to get the flavor I want...usually 1-2 oz. Then I add in enough 60 hops to get within 10-20 IBU of the total I'm shooting for. Since I use FWH, I seldom use a 20 min. addition, so the rest of the IBU come from 15 min. or less additions.