How to resolve differences in IBU calculations?

I'm making Jamil's Belgian Dark Strong Ale from "Brewing Classic Styles", which calls for 4% Hallertauer. The Hallertauers I've got are only 2.1%, which is no biggie. But when I use Beer Smith to work out the appropriate amount of extra hops to add, I notice a big discrepancy between Beer Smith and Jamil's recipe. Jamil's recipe says 2.42oz of 4% Hallertauer give 31 IBUs, whereas Beer Smith says 2.42oz of the same hops give 21 IBUs. Anyone have any suggestions as to which I should go with?

Jamil uses Reger formula for IBU calculations, maybe you use different - check program settings.

Spot on - Beer Smith was set to Tinseth (giving 21 IBUs). When I changed it to Rager, the same amount came out at 31 IBUs. Thanks Piotr!

Now I guess I should go and read what the difference between the different methods is...

The differences in the methods are simply calculations. I've heard Jamil say on one of his show that none are necessarily the best-- but you need to pick the one you want to use and stick with it so that you stay consistent. If you use Jamil's recipes, it seems like Rager would be the best for you to use.

Basic Brewing also has a podcast where they ran beers through all of the different calculation methods and then ran the beer through a machine that actually measures the real IBU content. None were right on, but some were definitely more consistent than others.

There was a thread on here that said one of the calculations was better for partial boils, while the other was better for full boils. Of course I can't remember which is which, so I guess I'm no help at all

Found it: https://www.homebrewtalk.com/showthread.php?t=73025&highlight=calculators