Say phosphate buffer, for a particular strength many protocols (including coldspring harbor, NEB, Labrat etc.) suggest that adding 'X' amount of monobasic and 'Y' amount of dibasic will give you phosphate buffer of you desired pH.

But at the same time there is a Calbiochem buffer booklet that suggests addition of water equal to 'X+Y' in addition to the above mentioned addition of two species.

As per my knowledge the first approach is right because in second approach I guess the molarity is halved.

Pls suggest.