I use Beer Tools Pro with their "basic" (standard) IBU calculator (the one that's used by default).
Beer Tools Pro by default doesn't use Rager or Tinseth. You can select from a number of curves and even edit them yourself if you want. There's also gravity and pellet hop correction graphs that can be loaded/edited. Neither is used by default.
Here's the window where you select what to use:
If you wan to manually edit the curve or create your own it looks like this:
More details here:
http://www.beertoolspro.com/wiki/Utilization
So the "basic" curve which is their default gives gives me 36.9 IBU.
Changing to the "Rager" curve gives gives me 40.5 IBU.
Changing to the "Tinseth" curve gives gives me 31.4 IBU.
There's also Mosher (24.4 IBU), Garetz (13.2 IBU), Daniels (52 IBU), and Fowler (22.3 IBU). You can create your own too.
As you can see, the IBU will vary greatly depending on which expert you want to believe as they all have different curves as they how "they" think hop bitterness is absorbed based on boil time. A range of 13.2 to 52 is massive!
I take them all with a grain of salt as there's a lot of variables involved including hop age too. I basically feel there's no such thing as too much hops near the end of the boil (last 0-5 minutes).
One of these days instead of adding all these hops at various times, I'd to try plopping in a full pound of hops at flameout and nowhere else and let it sit for 5 mins before chilling. I think that would be an interesting APA. I know some commercial beers only hop through a hop-back (basically the same thing) but then they have longer contact time usually and due to the large volumes the beer is often still (technically) boiling.
The only thing these experts have in common is that if you boil long enough, you'll extract all of the hop bitterness. They argue a bit as to how much bitterness is extracted total but they're fairly close. Where they disagree with each other is how fast hop bitterness is extracted over the first 20-40 minutes (the early part). That's where an all-late-addition recipe like this falls apart when it comes to IBU calculations and you'll see widely varying IBU numbers. Most beers have a bittering addition at 60 minutes which gives the beer most of its bitterness so most curves will give similar results.
So long story short: Don't put too much stock on IBUs. There are a dozen different ways to calculate depending on who you believe. The only way to truly know is to have the final beer analyzed.
Kal
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