In my limited testing, the 2 batches I had tested that were FWH measured 10% more IBUs that the same beer with only a 60 min. addition. But they didn t taste more bitter and a lot of the testers thought the same. There's no way you can put a number on it, though, since it's subjective sensory. And remember, an IBU is not always the same in terms of perception. It's all up to you.