We talk from time to time about diluting whiskey with water. It's a good idea when doing a comparative tasting to dilute the samples to a common proof. I tried to work out a formula for doing this but it just made my head hurt, so I went looking for one on the internet. I found this and it works. For maximum ease of use (especially if you are a math idiot like me) plug it into a spreadsheet application.
The formula is:
(amount of whiskey) x ((bottle proof/desired proof) -1) = amount of water to add