As the weather was pretty ordinary today, in Sydney, I spent the day indoors and I came up with this pretty simple calculator.
http://www.zip.com.au/~cswarren/BeerCalc.zip
If you feel inclined have a look and give it a go and let me know whether it is right, working, bug ridden etc etc etc...
If you want the source you are welcome to it on one condition:-
Don't take the p*ss out of the code

I have used several sources for the maths, although they look ok I am open to suggestions on better sources if the results you get don't tally with known good data.
Rabz