Most whisky connoisseurs would probably say that you should not add water to the whisky as you are adding something that the distillery did not intend to be added. Full flavoured and powerful.
If you enjoy your whisky with a bit of water, try to use distilled water which will be void of any flavour, when compared to city water which may taste of chlorine and alter the whisky, for the worse
How much to add you ask? Getting your whisky from 40% down to 35% or 38% is a great start. This may be just a few drops, but you would be surprised what a small reduction in alcohol content can do to get rid of the burn. I really enjoy whisky at 35%, but everyone is different.
If you work at a distillery and want to make sure your whisky has no off flavours and checks off all the boxes, cut your whisky down to 25%. This is low enough that almost all of the alcohol burn and flavour is gone, allowing flavours to show up that were being covered up by the burn. I wouldn' drink whisky this low in ABV, but it is always a fun experiment.
Adding Ice has the same effect as chilling beer or white wine. The colder liquid the less you can taste I like to use ice in my whisky every now and then as I find it interesting to see how the flavour changes over time, but don't leave it too long as it can become a watery mess.
Chill your bottle in the fridge, even throwing it in for an hour or 30 min in the freezer will do the trick. Add chilled water to your desired level of alcohol.
Chilled whisky is great because it is not too cold and whisky as it gets cold, gets more viscous and creates a wonderful mouthfeel. Adding water cuts down on the burn, but unlike adding ice, you get to control the dilution. This is probably my favourite way to drink Twelve Barrels.