The bitterness i get out of an 30min 80c hopstand is negligible i find. I just recently whirlpooled 60g of 18%aa Eureka hops for half hour above 80c and i thought that beer was lacking bitterness it was reading high 40s IBU. I set my utilization at 5%.
Mines more of a hop stand than a whirlpool to BTW with the odd stir every 5-10 minutes or so.
Any hopstand close to 80 C would be negligible since 80 C is about the cut-off point for isomerization. The length of time to drop from 100 C to 80 C could give additional IBU's.
Another very simple option if only cooling is involved, is to add to the boil boil length about half the time it takes to cool to 80 C (e.g. if actual boil length of the hops is 60 minutes, and the time from flameout to 80 C is about 10 minutes, set the boil length to 60 + 1/2 time to cool to 80 C, or 60 + 5 = 65 minutes).
The whirlpool or hopstand is more difficult to estimate since it depends on the temperature and length of time. A very simple estimate could be to add the result of the following equation to the boil length:
((Temp. Whirlpool in C - 80)/20) x (Length of Whirpool or Hopstand in min.)
For example, if the whirpool was done at 85 C for 16 minutes, then add:
(85 - 80)/20 x 16 = 4 minutes.
Thus you would set the boil length to 64 minutes instead of 60. In addition, you would also add an estimate for the time it took to cool from 100 C to 85 C as explained above.
Of course, this only accounts for the initial hops. Another estimate has to be added for the additional hops added after flameout. But since flavor/aroma hops are usually low alpha, it should not contribute too much in terms of additional IBU's.
