How much water you add to your dram comes down to a matter of personal preference, whisky age and ABV (alcohol by volume). The point of course is to maximize those aromas and tastes that the distiller has taken years to create.
Before you add anything, get your hands on a good glass like the Glencairn that was made for the whisky experience and second pour yourself some and let it sit out for around 10 minutes (old whiskies may need more time to open up).
Next you need to assess the ABV. For a whisky bottled at between 40%-43% I would simply leave it breathe and skip the water because you may risk watering it down too much at this percentage (think Glenfiddich, The Macallan etc).
The trend among whisky connoisseurs however is for something that is ‘cask strength’ or at least high enough that the whisky drinker can make the decision as to the final strength for themselves. This may be anywhere from 46% (Ardbeg 10) to around 63% (some versions of Aberlour A’Bunadh) and everything in between.
This is where some good quality cool water works its magic and all you need in addition is a teaspoon to measure with. At around 46% I like a 1/2 teaspoon and as I approach the 60% range it’s closer to a full teaspoon in your standard single measure of scotch. This is something you will have to play with and see what works for you.
Things can get a little tricky however, with a whisky that is quite old (let’s say 25-30 years) which can only handle a little water compared to your standard younger whisky that can take much more without the risk of ruining it.
Hopefully this provided a few helpful tips to help you navigate the world of whisky.