Run to Swim Equivalent

I have a question.

How do they set the first distance of the first run in a duathlon?

Like in the duathlon I did last week, it was a 1/2 mile swim for the triathlon, and a 2 mile first run for the duathlon. This weekend is going to be another 1/2 mile swim for the triathlon but a 2.4 mile first run for the duathlon. Is there some general formula the race promoters use that says something like this long of a swim is about the same (I guess output/energy wise) as this far of a run??