Isomerisation continues down to 79C. Hop stands at high temperatures have the potential to contribute significant bitterness depending on hop AA%, amount, time and temperature. So to answer .. negligible bitterness? Negligible, but not insignificant in a more traditional brew, definitely should be considered in something like a NEIPA. Calculate or not? Well this is a can of worms.
Short of laboratory analysis on finished beer all brewhouse IBU calculations and such when formulating recipes NEED to be accepted for what they are .. a fairly subjective measure of 'brewery bitterness'. As long as you work them out the same way each time and do the same sorts of things on brew day and your taste buds are the final assessor they are a useful way to allow you to formulate and adjust recipes. It doesn't matter if you calculate everything out at 10 or 100 IBU's as long as you know what you need to do to make that number half as much or twice as much and it works and your assessment of the final beer feeds back into it.
So that said does it matter? Is it any more useful than measuring with handfuls or similar? My point is don't get too caught up on it, but do think about how hops, even unboiled can contribute a bitter taste even if that bitterness isn't from isomerised alpha acids (theoretical IBU's aren't the be all and end all) and how proven techniques can be adapted (hop stands, transfer time, low temperature hop stands, whirlpools, hop backs etc) to get you where you want on your equipment.
Calculating theoretical IBU's is the usual AAU x U x 10 / V for me, except any hop stand and transfer time. The calculation doesn't work because there is no 'boil' time to get the utilisation for. For this I use 25% of the standard utilisation for the whole duration of both. I will stand up and say this isn't scientific, but it gets me where I need to be as I adjust from the same point each time. There is totally utilisation from hops in contact with 96-98C wort throughout the duration.
For beers where these numbers get slightly terrifying I will do a whirlpool and chill the wort down. I used to play around with different times and temperatures, but have pretty much settled on stirring them in and then starting the whirlpool and chilling immediately as it takes typically less than 30m to completely chill the wort, it is certainly 60C or less by 10-15m. For these I use 5% of the standard utilisation for the duration.
Another method I'll use as a compromise between the two is stirring them in at knock out, but immediately starting the transfer so they have only a short contact time. I'm less fond of this as I use leaf in the copper and like to allow some settling for filtration and I can always use the previous method, with some boil hops to bring up the bittering.
Anyway, that is what I do. I do quite a few things a bit differently sometimes, but usually have a reason for them. Rarely do 60m mashes, usually mash time is based on diastatic power and mash temperature depending on goals. Rarely do 60m boils, usually boil time is based on DMS potential of the grist and evaporation depending on goals. NEIPA's are usually no boil hops, 30-40m boil depending on nitrogen content of the malt, hops go in, whirlpool starts, chilling goes on, divert to FV once the copper temperature drops below 60C. Mash is usually 64-65C, simple brew day.
Body is build with a little dextrine malt, wheat, oats, whatever you like to use for protein though less than you'd think due to proteins and lipids will rapidly oxidise in the presence of oxygen, rarely bother with copper fining though yeast influences that choice. Sweetness is water treatment, limited adjuncts and hop choices. Final gravity is more a product of yeast selection, yeast is selected for esters, fermentation speed and attenuation, flocculation not so much. The real work is in the dry hopping and packaging.
It also depends on what school of thought you come from regarding these beers. Should they be a healthy 5.5-6.5% abv or 4%? Should they be hazy or cloudy? Bit of yeast or absolutely snided? Decent orange colour or pale yellow?
