Just curious, what formula do you use for FWH IBU? I currently have ProMash set to calc FWH as -50% utilization, which is effectively the same IBU as a 15 min addition.
Currently, I treat it like a 20 minute addition. This is what Brewer's Friend calculates it as, so that's convenient for me.
But I'm starting to waffle a bit on this. There is definitely a difference between "amount" and "quality" of bitterness. A FWH or hopstand addition definitely has a smoother quality
to the bitterness, and that is why treating FWH as a 20-minute addition makes sense. But the amount
of bitterness is still there. In the end I think you're better off thinking in the mindset of "X IBU's from a FWH plus Y IBU's from a bittering addition plus Z IBU's from my finishing hops are going to give me what I'm looking for in this beer", because they're not all equivalent. Simply adding together the IBU's from all your additions isn't necessarily going to give you the same results.
Here's what really cemented my thinking on this: I brewed an IPA over the summer using nothing but flameout hops with a long hopstand and dry hops. I sent it to a lab and it measured 98 IBU's. It only tasted like 60 IBU's to me, and it was a really smooth bitterness (similar to what I get from FWH). It was super drinkable, like an amped up APA. But I typically drink beer at the end of the night (i.e., not with a meal). The first time I had one of these IPA's with food I instantly tasted every one of those 98 IBU's. It was a bizarre experience. So, while the bitterness was far from enamel-stripping, it still totally blew out my palate for food. My new philosophy is "Not all IBU's are created equal".