The folks at Cambridge University are kinda smart. It’s sorta like the MIT of England. They put together this pretty nifty flight predictor. Click the link directly below to access the predictor.
Online Flight Predictor: Cambridge University Spaceflight Landing Predictor
We BHALDIans have also created instructions for using the predictor below. The instructions are specific to our May 20, 2012 launch, but if you are smart enough to launch a weather balloon surely you can figure it out how to apply it to your launch.