IBU Calculations and different formulas

I haven't worked with a lot of softwares. I recently put Brew Mate on my PC and I've played with Beercalculus as well. A curious thing I noticed with Beercalculus is the big differences in calculated IBU's depending on the hop fourmula used (the two formula's I compared were Tenseth and Rager). Brew Mate doesn't have a place to specify the formula. Beercalculus seems to use Tenseth as default, while a Pro Mash recipe I was looking at from someone's post used Rager as default. I'm curious as to whether or not anyone understands what goes into the different formulations and if there is a prefered formula to use (for those of you who formulate your own recipes regularly). Thanks.

I think there's general agreement that Tinseth is the most accurate (or at least the most extensively tested) and that Rager tends to over-estimate. The truth is that none of them will be particularly accurate across the whole spectrum of beers. When people actually get beers analyzed for bitterness, deviations of 50% from the predicted value aren't uncommon. The best thing you can do is decide on one correlation, use it exclusively, and let that determine how bitter your beers are *relative to one another*.