Originally Posted by IffyG
the key is to stick with one formula and use that so you can make comparisons between your own beer.
This is the correct answer.
I always use tinseth. It calculates IBUs much lower than Rager, since it doesn't value flameout hops as high. I know what 60IBU in tinseth translates for my beers. That's all that matters.
When I see a recipe that claims to be say: 80IBU, I plug it into my tinseth calculator to see how it compares to my beers. I don't trust the arbitrary IBU numbers that are thrown around. Just pick a formula, and stick with it.