For "dram" here read "what you put in the glass to start with". So, you start with as much as you like (call it two fingers), and after nosing and a bit of careful tasting undiluted, etc., work splash by splash towards adding a third as much water again as you have whisky in the glass. But keep testing along the way so you don't over-dilute. It's remarkable how different whiskies take more or less water - even different bottlings of the same whisky. And then again, sometimes you come across a whisky that drinks so well at cask strength that it's a shame not to just settle back and enjoy it as it comes (I have found Mortlach and Rosebank in particular seem to come across brilliantly at high or cask strength, as do quite a few bourbons.) As to what water you use, I think it's down to individual taste. I find tap water mostly works perfectly well, but so does still mineral water (Highland Spring, etc.). But not Vichy water - unless you like your whisky to have added notes of wet dog.