Not to speak for Denny but, usually when people reffer to GU/BU its total they are talking about. 1:1 would be like 1.060 to 60 IBU.
I start by deciding how much FWH I want to use (OK, you guys, don't start! ) That usually is 1-2 oz. Then I figure out how much to use at 60 min. to get in the neighborhood of the 1:1 BU:GU ratio Jess talks about.Is that ratio guideline for total ibus? Or just the bittering additions?
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The trouble is when it comes to high dose whirlpool additions. The science might say that no isolmerization is happening below X temp, but our mouths still translate those additions as some level of bitterness, though zero IBUs. If anyone doubts that, do a batch with no boil hops, and about 3 ounces of Simcoe at 150F for 30 min. There will be some bitterness. So, like any other ingredient, sometimes we can totally reduce hops to a numerical matrix. But experience will tell you what you like, don't like.
There are some good calcs out there that approximate the BU from whirlpool additions. We just incorporated some into our spreadsheet. Pretty simple actually, you just incorporate a modifier to the Tinseth utilization and viola.
I remember when I first started brewing, my early knowledge on hops was
1. Bitterness comes from isolmerized alpha acids. So obviously, there's no other way that hops could possibly make bitterness come into my beer. Check!
2. High alpha acid hops are for bittering. So you only can add them at 60 min, and they don't bring any flavor or aroma to my beer. Check!
3. Low alpha acid hops are for flavor only and you add them at 10 min before turning off the stove, or right as you turn off the stove for an advanced method called "Flameout". They don't add any bitterness, only flavor. Check!
4. Dry hops are the only way to get any aroma. Check!
Then I brewed a couple batches and start learning that there's more to it.
I use Brewer's Friend and it allows me to adjust utilization at whirlpool. For my tastes, experience, on my beers... I've gotten to where I glance at predicted IBU from my FWH/60 and then I ignore IBU from there on. It's all by experience and preference. When someone asks how many IBUs I say I don't know