The big changes from how it operates from BS2 are the removal of the whirlpool/steep utilization factor from the settings and folding the hop utilization into the temperature of hop addition.
As to the 'accuracy' of the IBU calculation, it is entirely a crap shoot. I know that Brad has backing information to have modeled the IBU calculation from whirlpool hopping, but given that the original model is really only accurate for the equipment used when collecting the data. [reference:
https://www.experimentalbrew.com/experiments/writeups/ibu-lie-kind and
https://www.experimentalbrew.com/podcast/episode-32-ibu-lie]
Without testing on your specific process and beers, you really don't know how it relates to actual test results. Further, the IBU test does not give you an accurate indication of bitterness perception. Proof of this is a good NEIPA where the calculations may give you a high number but the actual perceived bitterness is no where near what you would expect from that IBU number (and actual results if the brewers I have talked to are correct).
So, my recommendation is to base your values on bitterness perception. In my case, I brewed an American wheat beer using only whirlpool hopping with each batch containing the same amount and lot of hops each added at a different time and temperature. I compared this to my expectation of bitterness based upon several commercial beers ranging from low IBU to medium IBU to peg where I perceived the bitterness to fall in my process.
Using this as a rough guide, I space my addition of whirlpool hops based upon the time of the hop additions from flame out. I can relate the time of addition to the standard cooling rate of my kettle. I then assign the whirlpool temperature based upon the IBU contribution that correlates to my perception of bitterness at that addition time. It is not perfect, but it seems to work OK for my process.
