Well, there's where you get into a tricky area. I got interested in the subject because the hops corrections software was giving made everything too hoppy for my extract brews; personally I'd adjust a little bit (to account for the precipitation factor Palmer discusses in that podcast), but only maybe 10% more bittering hops and no adjustment on the 15 minute or less additions.
Unfortunately while the newer research shows that the old formulas are wrong, there isn't a well-accepted replacement yet; some people (including some really experienced brewers who know their stuff--e.g. YooperBrew) still think the old adjustments are fairly close, while others (including other experienced brewers who know their stuff, e.g. the measurements at BBR) would have you adjust very little or not at all.
Sumner and I have talked about this, and I'll further explain my own findings.
In a normally low IBU beer (15 IBUs), doing a full boil gave me an unexpectedly bitter beer. This was before I had Beersmith or heard that a full boil (or late extract addition) might change hops utilization. I remember saying, "This beer is twice as bitter as the last time I made it!" after my first full boil with it. Later, after I got Beersmith, I saw that the full boil changed the IBUs from 15 to 28. Which seemed about right to me, in the taste.
In a higher IBU beer, there was a much less noticeable effect. In an APA, I don't remember even noticing a difference! Possibly because there isn't an easy way to determine, say, 35 IBUs from 45 IBUs (as an example- I didn't calculate anything!).
So, when I discussed this with John Palmer (who originally said there was an impact of gravity on hops utilization), he blamed it on "break material" in the kettle. That is, the gravity doesn't affect hops utilization but instead the amount of break material does.
Also, when making hoppy beers, keep in mind that only so much of the hops oils can isomerize, and probably max out at 100 IBUs or so if you believe the current theories. That means if you're making a 75 IBU beer with a 2.5 gallon partial boil, well, you can't. Whatever IBUs you manage to get into the beer will be diluted by the top off water. So, if you do manage to get 3 gallons of 95 IBU wort, adding 3 gallons of top off water will definitely affect the bitterness. I'm not sure how accurate that is, but it makes sense that there IS a saturation point. If you do a full boil or a late addition of extract in this beer, you will have greater bitterness than a partial boil. And you'll be closer to the AG version of the same recipe, in that respect (boil volume, break material, gravity, etc).
All that to say this- for non-hoppy beers, where the bittering may be overpowering if you're making a beer like a cream ale, it won't hurt to reduce the bittering hops by 15% or so in a full boil or a late addition of the extract. But for a hoppy beer, particularly if it's a pretty high IBU beer, I wouldn't bother. Just be mindful of the ingredients and the desired effects.
) dry hopping - and might make up for a slight lack of bitterness, perceptually.
