Calculating IBU

Was gonna put this in Brew Science, but it seemed too noob to.

So my question regards what volume to use to find utilization while calculating IBU. And also can somewhat apply to calculating OG. The answer is easy and simple when doing All-Grain or if you boil all 5 gallons with an extract, but what about when do an extract and only boil 3 gallons and then add 2 more into the primary.

Obviously, you should use the 5 gal when calculating OG. A) because it makes sense, and B) because not using it gives you a ridiculously high OG.

But with IBU, I don't know what to do. Since the volume is calculated into utilization, and utilization only happens during the boil, you should use 3 gal. BUT, I have to believe that when you add 2 more gallons of water it dilutes the bitterness down (just like with the OG). So should you use 5 gal?

Any thoughts or equation modifications are more than welcome.

**And if a mod thinks this should be in another thread, feel free to move it, I was just worried calculating IBUs wouldn't be "hardcore" enough for that forum

You should use the SG during the boil (i.e. in 3 gal) to determine the utilization(s), and the overall 5 gal volume to determine the IBU in the finished beer. So when using a calculator, you could use 3 gal as the volume and multiply the result it gives you by 3/5=0.6 to get the overall IBU, or use 5 gal as the volume and multiply the recipe OG (in Plato) by 5/3=1.67 so that it will calculate the utilization correctly. Either method should give the same result.