This might help
https://byo.com/hops/item/2808-hop-stands
Alpha acids will continue to isomerize after flameout until the temperature of the wort reaches about 175 °F (79 °C). Homebrewers trying to calculate a beer's IBUs will need to guesstimate how much isomerization is occurring. The closer the wort is to 212 °F (100 °C) the higher the alpha acid isomerization rate. To do this, we can look to professional brewers for some guidelines. Ultimately, however, the thermal capacity of a professional 60bbl whirlpool vessel is quite different than 5 gallons (19 L) of homebrew, so the comparisons can only be rough guidelines at best.
Matt Brynildson of Firestone Walker Brewing Company says, "The fact that there is some isomerization (about 15% in whirlpool versus 35% in the kettle) of alpha acid means that not only hop aroma and hop flavor can be achieved, but also some bittering."
For Pelican Pub & Brewery's Kiwanda Cream Ale, brewmaster Darron Welch adds the beer's only hop addition at flameout. Welch gets about 25 IBUs from adding roughly 0.75 lbs./bbl (0.34 kg/bbl) of Mt. Hood hops at flameout then allowing a 30 minute whirlpool stage. This means that Darron is getting roughly 16% utilization on his 15 bbl system for a 1.049 specific gravity wort. As mentioned, in a homebrewers hop stand, the 5-gallon (19-L) kettle is going to cool much faster and therefore create lower utilization rates. Brad Smith, creator of the BeerSmith brewing calculator, gives this advice to homebrewers, "Something in the 10% range is not a bad estimate if hops are added near boiling and left in during the cool-down period."
From my own experience with extended hop stands in 11-gallon (42-L) batches, a 10% utilization rate for whirlpool hops seems reasonable.
