You're becoming quite the hop guy steve. Thats a good thing. I'll have to try your method once I get through lager season. Maybe a real simple IPA grain bill with a neutral 60 min of about 30% of the total ibus. Then the rest at 175. How long do you suggest that rest? 15 min? 30?
I love hops.
If I read your question correctly you are asking about the IBU contribution of a whirlpool addition. My assumption is that at ~175* there isn't much isomerization going on. So I go for all of my IBU's in my 60 minute addition. Beersmith however calculates that my whirlpool additions of 3 oz. each of Amarillo and Cascade will add about 50 IBU. I think the software assumes a higher temperature.
But I've got to say, I'm not cautious at all in this regard when dealing with AIPA's. The one I am brewing tomorrow is calculated to have 136 IBU. This is typical, and I have yet to have one I thought was too bitter. But I watch my pH and use a quality bittering hop (usually CTZ).